The entry of social movement studies into the domain traditionally occupied by revolutionary theory began as a series of modest observations that some of the features of revolutionary movements resemble those of social movements (e.g. Goldstone, 1998), and that revolutionary processes shared commonalities with the political processes seen in social movements (e.g. Tilly, 1995).

This convergence was unsurprising, as the two fields have always enjoyed a considerable overlap 5 , owing to the tendency of sociologists of revolution to occasionally moonlight as scholars of (or indeed participants in) social movements.

As time went on, even more ambitious attempts to fuse the two fields were made, and it was claimed that that revolutionary processes and social movements were in fact part of a single overarching category of phenomena known as "Contentious Politics" (McAdam, Tarrow & Tilly, 2001).

Captained by three highly experienced, world-leading scholars- Charles Tilly, Sidney Tarrow, and Doug McAdam- the 'Dynamics of Contention' (2001) project attempted to create a new field of study which paid careful attention to the relational dynamics, mechanisms, processes and episodes which recurred across a wide array of political phenomena, such as disaffiliated protests, elections, ethnic conflicts, civil wars, and terrorism.

Chief among the project's targets for agglomeration, however, was what its proponents perceived as "a largely separate specialty dedicated to the comparative study of revolution:" at that time the chief vehicle for fourth-generation revolutionary theory (McAdam and Tarrow, 2011: 2).


While McAdam, Tarrow and Tilly's desire for a relational approach mirrored many of the concerns of fourth-generation theorists, much of the work which the trio dedicated to the relational analysis of common mechanisms, episodes and processes occurring across political phenomena was not as warmly embraced as they had hoped.

While the project successfully expanded the territory of the social movement field to include revolutions and civil wars, its central attempt to encourage scholars to think about dynamic relational mechanisms rather than fixed variables was comparatively less successful (McAdam & Tarrow, 2011).

While the conceptual waters of revolutionary theory's fourth generation have been contested by efforts to reconceive revolutionary processes as a subcategory within social movement theory, efforts to analyse revolutionary outcomes as a subtype of democratisation have had similar effects.

Such tendencies have been evidenced in the attempts to recast the 1989 and 2011 revolutionary waves as democratisation attempts, rather than revolutionary cases (Huntingdon, 1991; Brown, 2013; Stepan & Linz, 2013).

This effort has proven more to the chagrin of fourth-generation theorists than had the Contentious Politics project, leading to allegations that "the link between democratisation and revolution [was] routinely abused by transitologists" (Lawson 2008, 157).


These shifts in the field have resulted in considerable ramifications for the cohesion of fourth-generation theory.

Casting an eye over the work published about recent revolutions leads one to realise that many such publications now couch their analyses not in terms of revolutionary theory, but in terms of either social movement or democratisation studies (e.g. Jumet, 2017; Stepan & Linz, 2013).

While there is much to praise about these two fields, and the considerable progress they have made while revolutionary theory has comparatively stalled, it cannot be denied that these fields pursue markedly different objects of analysis.

Revolutions are occasionally incident in democratisation processes, and they often feature social movements, but within the study of these two phenomena they are ultimately only fringe cases.

It is thus no surprise the knowledge produced from such approaches has tended to tell us much more about democratisation or social movements than it has revolution.


With the progressive displacement of revolutionary study by other bodies of theory, the field has borne witness to an increasing number of scholars who theorise revolution without engaging in self-conscious revolutionary theory.

Perhaps reflective of the comparative primacy of these approaches over traditional revolutionary theory is Allinson's desire to count Della Porta's (2016) Where Did the Revolution Go? Contentious Politics and the Quality of Democracy, within the bounds of his proposed new generation.

The book is instead a self-professed attempt to "read democratization through the lenses of social movement studies," (2016: 345).

Despite the merits of Della Porta's book (and there are many: particularly her concept of 'eventful democratisation'), it is inaccurate to assess the book as a work of revolutionary theory.

Indeed, Della Porta makes no claims to such an endeavour, and instead concludes the volume by specifying that her contribution was instead to identify "some mechanisms that convey the effects of social movements' participation in regime transition on some democratic qualities of the ensuing regime," (Della Porta, 2016: 345).


While questions about democratisation, social movements, and other facets of contentious politics assuredly remain important and relevant to the phenomenon of revolution, however broadly we cast our gaze around the periphery of revolutionary phenomena we must remember that the object of study around which revolutionary theory is situated is and remains revolution itself.


Rather than dismissing a fifth generation out of hand, it is fairer to say that revolutionary theory's fourth generation is decidedly imperilled: perhaps by its desire to cover too many variables, perhaps by its turn away from bold theorising, perhaps by its partial displacement by other fields, and perhaps by the passage of time.

Whether recent interventions will enable it to recover from this peril remains to be seen, but regardless of the success or failure of these efforts, the nascent seed of a fifth generation is already being sown.

John Foran (2003), a one time champion of fourth-generation theory has called for the field of revolutionary theory to more usefully orient itself towards the revolutions of the future.

Colin Beck (2018) has, through a series of incisive accounts and studies shown convincingly that present strategies of inquiry have stalled.

As Allinson (2019) notes, we already have a suite of cases which disrupt established understandings, and as Lawson (2016) has convincingly argued, revolutionary theory is in dire need of theoretical reorientation.


Efforts to sustain fourth-generation theory would be much better spent cultivating the germinal ideas and discomforts of the fifth generation yet to come.

This demands that we reward inventive scholarship, embrace novel projects and theories, and do away with ontological or methodological purism.

But, what might a forthcoming fifth generation look like? Perhaps this is beyond the scope of any of us to predict at present, but I should like to at least provide a sketch of the direction in which things may be going.

I have a number of suspicions as to where the nascent fifth generation will take us, chiefly informed by my interactions with fellow colleagues, graduate students, and those who we might cautiously call 'practitioners'.

In light of these exchanges, I am hopeful that we may well see a return to grander theorising than the kind seen in the fourth generation.

In order to advance past our field's current impasse, we need a return to the kinds of ambitious comparisons, and innovative, in-depth theoretical accounts which have consistently characterised landmark work on revolution.

Taken together, the past thirty years of scholarship have tended to show that concerns for multivariate breadth and processual partiality have been less conducive to scholarly progress than have past preferences for explanatory depth and empirical diversity.


As a relatively mature field, there is no reason that revolutionary theory cannot incorporate both kinds of analyses: in depth case studies and comparisons of differing cases to theorise the operation of important elements of revolution in a bold and compelling fashion, and larger scale meta-analyses to show how these elements interact with scientific precision.

Not only is the opportunity ripe to give some order to the diffuse and partial analyses of the fourth generation, but also to revisit and update the contributions of the prior three in view of a more expansive object of study, and a more assertive theoretical bent.

A crushing absence of difference-driven inquiry in past decades (Beck, 2018) has also left plenty of room for novel comparisons stretching across time, space, and culture, through which new insights might be achieved.


There are also arrays of unsolved problems and theoretical lacunae which are ripe for attack from a new generation of theorists.

It is certainly true, for example, that the coming generation of revolutionary theorists will need to account for the counter-revolutionary and authoritarian outcomes which arose from the 2011 revolts (Allinson, 2019).

As Foran (2003) has emphasised, both the ends and endings of revolutions are areas much neglected by present-generation scholars.

Revolutionary theorists have traditionally been very good at stating the relation between pre-revolutionary conditions and subsequent revolutionary challenges, and fourth-generation theorists have extended this analysis to the relation between revolutionary contexts and ongoing challenges elsewhere.

However, very little has been done to clearly link the shape of revolutionary challenges with subsequent post-revolutionary outcomes (be they positive or negative), while retaining a perspective that looks beyond organised movements or static state boundaries.


This is not the only context in which revolutionary theory will need to look beyond revolutionary movements in its unit of analysis.

Indeed, a fifth generation of revolutionary theory will need to look 'beyond the movement' in several other important ways if it is to capture the full variety of revolutionary phenomena.

Not least, we will need to look beyond the organised networks of revolutionary elites and their followers, and again consider the role of the masses: not as a disordered mob, but as an array of complex individuals responding to intense socio-political phenomena and drawing on a rich palette of imaginaries (Selbin, 2019).

As Beck (2011, 2015) has affirmed, we will also need to consider the role played by revolutions in transforming societies other than those in which they initially occur. And finally, I would contend that we will need to reassert the importance and veracity of decidedly revolutionary theorising, studying revolutions as socio-political events that are not always reducible to merely more extreme forms of social movements, processes of democratisation, or a subcategory of 'contentious politics'.


Finally, reflecting on the concerns of revolutionaries themselves, I sense that they hope- but do not expect- that the next generation of revolutionary theory will be more decidedly engaged with revolutionary practice.

Since the end of the Jacobin-Bolshevik revolutionary epoch (1789-1989), scholars have become all the more content to regard revolutions with dispassionate interest, watching these bloody, brutal and dramatic social processes take place with a steely gaze before integrating them into their models or dataset for 'productive' analysis.

But revolutions are not toys, and they are not scholarly miscellanea.

They are the bloodied, broken corpus of the body politic in conflict with itself.

As such, we should aspire that future revolutionary theory might go some way toward ensuring that these processes result in collective emancipation, rather than merciless annihilation.


A recent exchange between Allinson (2019) and Abrams (2019) on the current state of revolution theory rests on the assumption that the generational, backward-looking view of revolution studies is also a fruitful way of thinking of the field’s present and future.

We argue, in contrast, that while a generational approach has important benefits, it also contains shortcomings that may lead the future of revolution studies in less fruitful directions.

We examine where an overreliance on generational thinking has led us, and provide an exploratory sketch of how we can begin to move beyond generational thinking and imagine a new future for the study of revolution.

It is with great interest that we read the exchange between Jamie Allinson (2019) and Benjamin Abrams (2019) on the state of revolution theory in the social sciences, and not just because our works are among those that represent, for Allinson, a failed fifth generation or, for Abrams, a self-monitored fourth generation.

We do not seek to quibble with their insights and critiques.

Rather, we propose that their debate has taken place on false premises.

Jack Goldstone’s (1982, 2001) formulation of generations of revolution theory has been an invaluable orientating tool for virtually every student that has entered the field over the past four decades.

However, it has also led to the establishment of an overly neat and consolidated perception of the field.

Goldstone’s historical-theoretical review of the literature has become reified as the actual history of the subfield in a way that at times limits our ability to assess what has been discovered about revolutions and, more crucially, prevents us from seeing where the path ahead lies by locking us into the idea that the future of revolution too will be constructed along generational lines.

Hence, we believe that Goldstone’s generational framework has been taken out of context and transformed beyond its original aim by subsequent generational reviews of the field.

Rather than being a tool for understanding and consolidating past gains, it has become the only way authors believe they can structure the field’s present and future.

By way of critique of the Allinson and Abrams exchange, we propose to take the field beyond generational thinking.

We argue that a persistent focus on generations is misguided as debates in revolution research actually occur laterally among contemporaneous studies of revolution, rather than sequentially between reified paradigms.

This is partially a consequence of mistaking touchstone texts for generations of scholarship (see for instance Ritter’s (2015: 9-12) review of the literature, which Beck (2020) has thoroughly dismissed).

As such, we have lost sight of our accumulated knowledge over the last one hundred years.

In fact, the basic contours of what revolution scholars should study, how they should study it, and how it should be explained is largely settled.

Consequently, it is time to imagine a new future for the study of revolution.


While not the first article to seek to make sense of subfield (see for instance Stone 1966), no essay has had more impact on the way scholars think of the development of revolution theory than Goldstone’s (1980) overview of the early to mid-20th century.

Updated two decades later (Goldstone 2001), Goldstone proposes that the study of revolution has come in the form of generations, where a general understanding of what was to be explained was widely shared by scholars who had common methodological and theoretical approaches.

Goldstone identifies an early-20th century first generation, which he terms “the natural-history school” (Goldstone 1982: 189), that focused on the sequential progression of the revolutionary process in classic cases of revolution, such as France in 1789.

In response, a second generation of scholars sought to place revolutions in the field of collective political violence, focusing on more general theories that connect the individual, psychological causes of discontent with social unrest.

Since Goldstone’s initial formulation, this approach has become known as the generation of social and psychological strain theories.

The third generation, termed “the structural-theory school” (Goldstone 1982: 189), moved the problem towards understanding why some revolutionary episodes succeed and others fail.

This approach has become most associated with Theda Skocpol’s (1979) state breakdown theory to the extent that the two are largely synonymous.

Most recently, theories of revolution for the past 25 years have been seen as belonging to a fourth generation that has an epistemology of conjunctural causation, dynamic eventful analyses, and an ever-expanding universe of factors under consideration (see Lawson 2019).

It is here where Allinson (2019) and Abrams (2019) differ.

For Allinson, the fourth generation represents a return of agency-centered explanations that was eclipsed by a short-lived “fifth generation” examination of unarmed revolutions and changes to political regimes.

For Abrams, the current focus on revolutions like the Revolutions of 2011 is a continuity of fourth generation approaches, and thus a fifth generation is yet to develop.


One obvious problem with these accounts is that generational thinking has obscured the long-standing roots of different approaches and the remarkable re-occurrence of theoretical imagery across different scholarly eras (Beck 2020).

We highlight a few examples.

First, the natural history school never entirely died out, as is seen in Sohrabi’s (1995) account of revolutionary eras.

More recently, the turn towards theories that highlight the salience of how actors interact during the course of revolution (Ermakoff 2015; Kurzman 2004b) shares much of the epistemology of natural history and might be considered its 21st century heir.

Second, a focus on the state as the primary arena, and actor, for revolutionary claims predates Skocpol by over a century.

Alexis de Tocqueville’s (1856) account of the French Revolution is remarkably state-centered and can be considered the first modern work in the social science of revolution (see Goodwin 2001).

Structural theorization also re-occurs across the so-called generations, whether in Merriman’s (1938) comparative history, Johnson’s (1966) modernization model, or Lawson’s (2019) mechanistic account.

Third, even strain theories of revolution have re-occurred, as grievances are a central point of some natural histories (Sorokin 1925) and a feature of some recent work (e.g., Beissinger 2013; Nepstad 2011; Snow and Moss 2014).

And, finally, the generational account has obscured the contributions of Marxist theories of revolution that date from the 19th century but live on in pieces into the 21st (e.g., Boswell 2004; Foran 2005).

Goldstone’s initial formulation was in equal parts exhaustive and crisp, thus inviting subsequent authors to adopt his framework rather than seek their own.

As a result, it is uncommon to find reviews of the revolution literature that do not follow a generational typology.

Because of this vast Goldstonian influence, the metaphor of generations has at times limited our view of the field’s actual progression, and, importantly, how we collectively envision that development.

In our view, a complimentary way of approaching the field of revolution, which might remedy some of the unintentional shortcomings of the generational view, is to think of it as a series of recurrent debates.

What should be the object of study: cases of successful transformation or cases of various forms of collective action? What should be the method of study: deep understanding of a particular case or general patterns in a universe of cases? What should be the core theoretical imagery: the distal causation of underlying conditions or the process of revolution itself? At any given point, there were always advocates for both sides of a question.

But the metaphor of generations inaccurately suggests that each era of scholarship breaks with its predecessor rather than builds on it.

For instance, the natural historians did not so much leave aside whiggish history as much as they put it to use in service of the emergent science of society.

Strain theorists pioneered new models of collective behavior, synthesizing previous observations into general laws.

And structuralists were arguing for or against Marxist theories of revolution as much as general models of collective violence.

In short, it is perhaps more fitting to think of debates as occurring laterally—that is, within a “generation”—than thinking of them as occurring sequentially—that is, across generations.

Another potentially unfortunate and inadvertent consequence of generational thinking is that it may promote the idea that past generations have been proven to be “incorrect.

” The consequence of this is that the baby—specific insights—is thrown out with the generational bathwater, perhaps due to a scholarly fear that returning to past understandings indicate both a lack of knowledge of the field’s history and, worse yet, an absence of theoretical vision.

In other words, key insights from past generations may become damaged goods that one should preferably avoid engaging, other than in one’s generational review of the literature or as straw humans to hack down.

The result of such an approach is detrimental to our collective effort to understand revolutions, as theory construction often occurs from the synthesis of pre-existing ideas, insights, and concepts.

This leads to our next point.

Generational thinking has also led to the canonization of some works and the forgetting of others.

Each so-called generation has a book that is taken to be emblematic of an approach.

As with any canon, this is constructed after the fact.

For instance, the enduring popularity of Brinton (1938) is reflected in two recent titles: The Anatomy of Revolution Revisited (Stone 2013) and Anatomies of Revolution (Lawson 2019).

This is somewhat ironic as it is, in fact, Edwards’s (1927) The Natural History of Revolution that gave the school its name, to say nothing of the largely forgotten contributions of Sorokin (1925) and Merriman (1938).

Similarly, a reliance on Gurr (1970) as a stand-in for strain theory in the revolution tradition has obscured the more erudite models of Smelser (1962) and Johnson (1966).

And the dominance of Skocpol (1979) has led to the marginalization of other contemporaneous structural accounts, such as Paige (1975), and mistakenly made the third generation equivalent with state breakdown theory.

Taking these works out of the larger conversations that were a part of their production has contributed to the reification of generational thinking.

What is unique about the last few decades of revolution theory is that they have lacked such a touchstone text.

This is where we believe Allinson and Abrams got off on the wrong foot.

They repeat the common rhetorical move to pin the “fourth generation” not to empirical work, but a literature review; most commonly Goldstone (2001), but also Foran (1993).

That there is no recent work to point to as emblematic of recent theorization is taken to illustrate a subfield in disarray, and thus the fourth generation is either a completed failure (Allinson) or a work in progress (Abrams).

But canonization is itself a social construction.

The fact that the generations exist in the first place, we could argue, is that a few texts are seen to be similar to a canonical text published around the same time.

The key here is are seen.

Take, for example, Chenoweth and Stephan’s (2011) Why Civil Resistance Works: The Strategic Logic of Nonviolent Conflict. Were this to be canonized as emblematic of early 21st century revolution studies, we would think that the central break with third generation structural theory was an agentic turn towards movement strategy.

However, this would leave aside the numerous insights that the last thirty years have brought.

For instance, the role of leaderships in revolutionary movements (Selbin 1993), the impact of culture and ideology (Bukovansky 2002), the turn from revolutionary change to reform (Lawson 2005), causation of international conditions (Beck 2011; Foran 2005; Lawson 2015; Ritter 2015), and so on.

Without a clear symbol of “fourth generation” thinking, it is easy to mistake the era as a failure or argue that it has already been replaced.

The very lack of a canonical text is, in fact, an advantage for recent scholarship.

Here, our take is a variant of Abrams’s argument, as we believe the diversity of recent approaches is a welcome development.

In order to break with generational thinking, we need to get away from the idea that certain texts are linked to certain ways of thinking about revolution.

Rather than an elitist conceptualization of knowledge accumulation—book X is the best one that occurred at time Y—we argue that each scholar should make their own determination of which studies to base their work on.

The field will develop best if it is turned into an open playing field where each text and each insight is assessed on its own merits, rather than compared to touchstones.


A final consequence of generational thinking has been the rhetorical move, in literature reviews and theoretical frames, to suggest that a dramatic break with the old is required for something new to be learned.

Allinson’s (2019) critique is very much in this vein.

When we take this rhetoric seriously it is easy to conclude that there are no consistent findings within the study of revolution (see Kurzman 2004a). But, in fact, there are.

We do not seek to repeat a catalog of findings here as we have already done so elsewhere (see Beck 2017, 2020; Goldstone and Ritter 2018).

Rather, we suggest here that the central questions of the field that animated 20th century debate—what to study, how to study it, and how to theorize—mostly have been settled.

The first point of debate in the first half century of revolution theory was what the object of study should be.

On the one hand, revolutions could be taken to be only the cases of political and social change that were world historically important (Brinton 1938; Edwards 1927; Huntington 1968; Skocpol 1979).

On the other hand, revolution could be taken to be one form of contentious politics more broadly and placed alongside riots, social movements, terrorism, civil war, and so on (Gurr 1970; McAdam, Tarrow, and Tilly 2001; Smelser 1962; Tilly 1978).

After nearly a century of the social scientific study of revolution, such diametrical poles seem absurd.

No longer do scholars only study successful transformations of state and society. And no longer do scholars see revolution as on a continuum of the irrational exuberances of collective behavior.

Rather, revolutions are seen as a distinct process of collective action, where success and failure are outcomes to be explained rather than criteria for inclusion or exclusion in a study (e.g., Foran 2005; Goldstone 1991; Ritter 2015; Tilly 1993).

And avowed revolutionary movements do exist, but the various types of contention that can result in revolutionary challenges need to be explained (e.g., Beck 2015; Chenoweth and Stephan 2011; Goldstone and Ritter 2018).

In short, the object of study is clear to contemporary scholars: attempted and successful shifts in, at the very least, regimes or political structures that occur through irregular mass mobilization.

The second debate that animated prior scholarship was the method of study.

Should the scholar examine cases for particular historical knowledge (e.g., Kurzman 2004b; McAdam et al. 2001) or should they seek general laws of revolution (e.g., Gurr 1970; Huntington 1968)?

The epistemological debate here obscures what is essentially a methodological dispute.

On the one hand, some preferred historical case comparative modes of inquiry and others preferred cross-national statistical analysis.

Modern revolution studies, for the most part, does not make this false choice.

For instance, Goldstone (1991) and Gould (1995) have shown how careful historical analysis can be combined with innovative quantitative study to leverage the advantages of both approaches.

Even among recent statistical studies (e.g., Chenoweth and Stephan 2011; Kadivar 2018), case comparison is a common compliment to the average effects within a population.

This is not to say that all studies of revolution employ mixed methods, but rather that methodological approaches are not mutually exclusive.

Debates over method are thus of a time that is passed.

This, however, has not settled the larger epistemological question: what should theories of revolution focus on? This leads us to the third point of debate.

Finally, there can be disagreement over the core theoretical imagery of revolution studies.

On one hand, should revolution be understood as resulting from broad causal conditions (e.g., Foran 2005; Goldstone 1991; Skocpol 1979) or seen as a dynamic process (e.g., Ermakoff 2015; Kurzman 2004b; McAdam et al. 2001)?

In essence, this is an epistemological debate over the role of structure and agency in revolutions.

This debate is not settled, and we should not expect one subfield to solve the structure-agency problematic.

We suggest, however, that a feature of recent scholarship is to leverage both lines of thought.

For instance, both Lawson (2019) and Ritter (2015) combine a backdrop of causal conditions with an account of how particular cases of revolution unfolded in the streets.

Now it is possible to prefer structure, as does Ritter, or lean slightly more towards agency, as does Lawson, but it need not be to the exclusion of the other.

With time, we may in fact see that the advantage of the so-called fourth generation (Abrams 2019) or failed fifth generation (Allinson 2019) was to sidestep theoretical disagreement in favor of empirical work.

As with methodology, preference for particular causal imageries may just be an academic parlor game that has gotten in the way of knowledge accumulation (see Beck 2018).

Related to this final point, generational thinking constitutes a particular obstacle to the study of revolutions, due to the fact that theorization has long been perceived as central to the subfield.

This has meant that reviews of the literature tend to obscure the distinction between empirical findings and theoretical contributions.

Put differently, what we think we know about revolutions has historically been framed as pre-packaged theoretical solutions advanced by individual authors to revolution-related puzzles that “fit” with other such individual insights produced at a similar point in time, rather than empirical insights that can serve as inspiration to and be adapted by other scholars.

By suggesting we have not achieved a substantial advance until some consensus on a new generation emerges, we risk losing sight of important, untheorized empirical insights that might quite plausibly give rise to future theoretical breakthroughs.

In sum, if we free ourselves of the metaphor of generations of revolution theory, then we are able to see not only the continuity of debates over the decades but their (mostly) successful resolutions.

Contemporary revolution studies have a consensual object of study, a pluralistic methodology, and a potentially fruitful empirical core.

This success, and we are comfortable calling the last century of the social science of revolution successful, should not be obscured by a rhetoric emphasizing the need to throw out the old and remake the field anew.

If there is something to generational thinking, it is only that: calls to do away with prior scholarship seem to re-occur every 20 years or so.

Our point thus far has been that the metaphor of generations of revolution theory creates a false premise on which we debate progress or the lack thereof.

We see previous work as caricatures rather than as products of a larger scholarly conversation.

And we lose track of what has been settled and what remains to be explained. If we free ourselves from wondering what the fourth generation was or is, and what the fifth generation is or will be, we can begin to chart a more fruitful future for the study of revolution.

What any field needs, and what revolution theory currently requires, is novelty.

Rather than musty generational debates, we need creativity in theory, empirics, and resulting implications.

In ideal-typical terms, there are two dimensions to any study of revolution.

On the one hand, research can be theoretically-oriented—focused on developing new understandings—or empirically-oriented—focused on explaining substantive phenomena.

On the other hand, research can have universal implications for understanding revolution or more specific, particularistic claims about certain cases.

Figure 1 presents these dimensions as a two-by-two table, highlighting four potential sources of novelty in the study of revolution with examples from the last two decades of research.


First, there is novelty that results from theory-oriented research that has universal ambitions.

Work here tends towards theoretical parsimony combined with the examination of well-known cases.

For instance, Foran (2005) argues that recent Third World revolutions can be explained by the combination of five factors—dependent development, exclusionary regimes, a culture of opposition, economic downturns, and a world systemic opening.

In another example, Lawson (2019) searches for recurrent mechanisms across executed and attempted cases of revolution ranging from the early modern world to the present.

He concludes that there are eight processes that occur across revolutions—inter-social relations, loyalty or disloyalty of coercive forces, regime type, state stability, innovative oppositional strategy, factionalism among revolutionaries, continued contestation after the fall of a regime, and limited economic gains for the post-revolutionary state.

Both of these works share the theoretical ambition to explain revolutions generally and the empirical incorporation of many cases.

As such, they generate novel insight into the patterns of revolutions and their outcomes.

At the opposite pole is research that is empirically-oriented explanation of particular instances of revolution.

Here the objective is the incorporation of new cases into the revolution literature, whether from current events or the successful recovering of past history.

For instance, as the Arab Revolutions of 2011 spread, scholars took voluminous notice of the cases without necessarily making broad claims about the nature of revolution.

For example, Ketchley (2017) examines Egypt in 2011 and afterwards using well-known tools of analysis to explain why and how protestors were able to oust Mubarak and unable to consolidate their gains.

Novelty here comes from the success of affirming the membership of an event in the fold of revolution studies.

The approach contributes to the field by expanding the case universe.


Third, other work seeks to explain new phenomenon rather than cases, with more universal import.

A primary example of this approach is the civil resistance literature.

For instance, Chenoweth and Stephan (2011) build on this approach to establish through statistical analysis and case examination that in the modern world unarmed strategies of resistance tend to have more success than armed ones.

The goal for works of this type is to ask a new question—what are the causes and consequences of a relatively unrecognized aspect of revolution?—and offer answers that have implications beyond the cases at hand.

Novelty thus comes from the examination of new dimensions of revolutions.

Finally, a fourth source of novelty is to create entirely new ways of understanding that focuses on particular instances of revolution.

Here, the goal is usually not abstract parsimony but theoretical richness.

Two recent examples of this approach are Selbin’s (2010) consideration of the role of stories and narrative for revolutionary participants and Ermakoff’s (2015) appraisal of how contingent decisions by individuals can radically change the direction of revolutionary events.

Both consider the particulars of their examples, but provide new theoretical lenses for the field.

Notably, they both also suggest different methodologies than are commonly used—oral history and decision-making trees.

Novelty, here, thus comes from theoretical and methodological innovation.

Sometimes a field is moved forward by parsimony, sometimes by complexity.

Sometimes it moves forward through events, sometimes through patterns.

Thus, all of these approaches to revolution should be encouraged and embraced.


Charles Tilly famously told his graduate students that there are three types of academic work: joining an ongoing conversation, changing the direction of a conversation, and starting a new conversation.

The first is lower risk, and lower reward.

Work on revolutions that explicitly places itself in generational debates is of this ilk.

The second is medium-risk and medium-reward as it may or may not shift a debate.

New abstract models of revolutions and examination of new cases are typical of this approach as they build upon the old rather than create anew.

Finally, the third sort of conversation is high-risk and high-reward as it can spur new paradigms.

Research on new phenomena and new theoretical lenses are of this sort. On one hand, scholars of unarmed resistance have had great success at creating an entirely new conversation at the intersection of sociology, political science, and international relations.

The flurry of work that Allinson sees as a fifth generation is proof of that.

On the other hand, Selbin and Ermakoff have challenged us all to think of revolution differently.

The last century of revolution studies certainly has been a very long conversation.

As such, it is unsurprising that we often repeat ourselves. But with a more accurate memory of what has been said by whom, to whom, and for which purpose, perhaps we can stop arguing over generations and resume debating ideas and phenomena.

Only this way will we push our dialogue forward for another hundred years.


Revolutionary theorists are currently immersed in a critical debate about the future of the field.

Allinson has argued that a fifth generation of revolutionary theory has passed us by without our noticing, while I have contended that it is revolutionary theory's fourth generation that is decidedly imperilled.

Ritter and Beck contend that we should reject the very idea of theoretical ‘generations’, and instead think of progress in revolutionary theory as a series of ongoing and settled debates about certain key topics.

The pair contend that revolutionary theory has reached a consensus on two core debates: defining our object of study and determining appropriate methods.

Contrary to this position, I argue that while there is much to praise about rejecting generational imagery, doing so necessarily entails that we also critique the self‐proclaimed ‘fourth generation' with which such imagery is intertwined.

Furthermore, I argue that there does not yet exist consensus among revolutionary theorists about a single definition of revolution, or on the question of which methods to use.

Finally, I call for a regeneration of revolutionary theory which moves genuinely beyond the generational mythologies of the past.


Over the course of the past decade, the intellectual project by the name of ‘Fourth Generation Revolutionary Theory' has been principally sustained by a trio of scholars.

George Lawson, Colin Beck, and Daniel Ritter, the collective heirs to a project primarily initiated by John Foran (1993) and Jack Goldstone (2001) some decades earlier.

Breaking with Lawson (2016 114; 2019), who has recently called for scholars to “fulfil the promise of fourth‐generation approaches to revolution,” Beck and Ritter (2021) now conversely argue that we should not only dispense with the notion of fourth generation revolutionary theory, but also the premise on which the self‐proclaimed ‘Fourth Generation' was built: the story of the prior three.

Though it once constituted the cutting‐edge of revolution studies, there is by now much to welcome in moving away from the embattled fourth generation endeavour.

I have previously written that Ritter and Beck's “attempts to provide life‐support to the otherwise faltering fourth generation” might “eventually lead to their breaking with the fourth generation project,” (Abrams, 2019, p. 381).

This eventuality has now come to pass, and the self‐identified ‘Fourth Generation’ has in essence passed with it.

Moving beyond such an overly elongated period of scholarly hand‐wringing has the potential to usher in a much needed rejuvenation and regeneration of revolutionary theory.

But as we put the fourth generation project to rest, what can we do to ensure a more fruitful future for our field? In what follows, I welcome the rejection of the generational paradigm, and the fourth generation project to which it is inseparably linked.

I nevertheless contend that the ‘fourth generation of revolutionary theory' can still be meaningfully distinguished from the narrative history of a prior three.

Rather than serving as a conceptual shorthand for a particular period of revolutionary study, the ‘fourth generation’ was instead a self‐conscious programme for the study of revolution, advanced by a series of distinct actors.

This, in turn, renders critique and discussion of the fourth generation project a meaningful exercise.

I also critique the argument advanced by Beck and Ritter (2021) that – in place of the generational narrative – we should instead depict revolutionary theory as having reached a consensus on two core debates: defining our object of study and determining appropriate methods.

I argue that there is neither consensus about a single definition of revolution, nor on the question of which methods to use.


Breaking with the fourth generation paradigm to which they once ascribed, Colin Beck and Daniel Ritter's recent article, Thinking Beyond Generations: On the Future of Revolution Theory, has made a convincing argument that progress in the field of revolutionary theory would be best served by discarding the concept of theoretical generations altogether.

This is a notable break from their past approach to the subject.

Beck, for example, once considered it “clear” that “the social science of revolution has undergone notable generational shifts in theory and the phenomenon of study,” (Beck, 2017, p. 170) Ritter similarly explicitly deployed a generational account of revolutionary theory in his landmark volume on international politics and unarmed revolution, The Iron Cage of Liberalism, (2015 9‐10).

While Ritter and Beck were once counted among the fourth generation school, they now eschew such thinking and instead encourage us to instead think ‘beyond generations’.

In their article Ritter and Beck level a number of productive critiques against overreliance on the notion of theoretical generations.

They note that while there is certainly some utility in the generational framework, it is outweighed by the cognitive constraints it places on our understanding of the field, and the hidden knowledge obscured by generational thinking.

While it has long been acknowledged that “classification of theories by generation is problematic,” (Foran, 1993, p. 1), Beck and Ritter skilfully draw attention to the great many scholars whose work has been eclipsed by the generational imagery derived from Goldstone's reviews of the field, and highlight a variety of contemporary scholars whose analyses do not fit into the ‘fourth generation’ programme.

Their analysis clearly demonstrates that refusal to move beyond an overly strict conceptualisation of theoretical generations has had an unhelpful impact on our thinking to the extent that today “generational imagery has obscured as much as it has illuminated.” Fortunately, there are many useful alternatives to generational framings.

Foran (1993:1) once suggested that “the best alternative is to group… theories by theme or approach,” and Ritter and Beck make a good case for recasting the history of revolutionary theory's early development in terms of a series of scholarly debates.

We might also seek to reframe what we have called ‘generations’ as intellectual fashions, thereby preserving the observation of rising scholarly tendencies over time without subsuming pathbreaking work into a single rubric.

There are many compelling reasons to explore at least one of these alternative frameworks where the history of revolutionary theory is concerned.


Despite the usefulness of their central proposition, I would certainly challenge Beck and Ritter's claim that the debate between myself and Allinson – about the fourth generation and its potential successors – “has taken place on false premises.” Regardless of the veracity of past reviews of the literature, it is impossible to deny the existence of an intellectual project by the name of ‘fourth generation revolutionary theory.

’ It is precisely this intellectual project – and the potential for a break with it – that formed the central topic of our debate.

What have been commonly referred to as the first three generations of revolutionary theory were in fact a set of historical cases drawn‐up by Goldstone (1980; 2001) in order to review the general changes in discourse between key scholars over various prominent topics in revolutionary theory.

Unlike the facets of past literature reviews that constitute ‘Generations’ 1‐3, the self‐professed ‘Fourth Generation of Revolutionary Theory’ was not an attempt to categorize the past, but an ambitious attempt to propose a new future.

This was a distinctive intellectual project in which an array of academics participated, and others declined to.

It was actively and self‐consciously cultivated and maintained by a network of scholars (e.g. Beck, 2017; Emirbayer & Goodwin, 1996; Foran, 1993; Goldstone, 2001; Lawson, 2016, 2019; Ritter, 2015), having been introduced in Foran (1993), Emirbayer and Goodwin (1996) and Goldstone's (2001) various manifestos for a fourth generation and continued in Lawson's (2016) more recent intervention on the subject.

A key part of the fourth generation's intellectual programme was the self‐identification of its participants as members of a new ‘fourth’ generation of revolutionary theorists.

This was grounded in a generational teleology that presented the self‐proclaimed ‘Fourth Generation' as the summation of three past ones identified by Goldstone (1980).

By its own reckoning, the fourth generation project was to be a coming‐of‐age for the study of revolution, after which scholars would overcome the tendency for single‐factor analysis, and instead adopt a holistic, conjunctural approach.

It was at once “a self‐conscious canon,” and “a project to be realized” (Lawson, 2016, p. 107).


As has now been well documented, the promise of the self‐proclaimed ‘Fourth Generation' theoretical project never truly came to fruition, despite some highly impressive initial progress by its champions, and valiant later attempts to redeem it by subsequent scholars.

Its troubled history shows that Ritter and Beck are right to identify the pernicious qualities of generational thinking.

As I have written previously, it was “the coincidence of the idea of theoretical generations with the emergence of a self‐consciously ‘fourth' generation” that encouraged the “peculiar tendency towards introspection” that plagued the fourth generation project (Abrams, 2019, p. 381).

Such a tendency, as Ritter (2019:106) has noted, has prompted many scholars in our field to become “unnecessarily modest, anxious, and…too self‐limiting.” There are certainly benefits of thinking beyond generations, but doing so does not permit us to erase the fourth generation from existence.

If we are to seriously consider dispensing with the concept of generations we must distinguish between the hypothetical three generations of revolutionary theory which Goldstone popularised and the proposed fourth generation which he, Foran and Lawson consciously advanced.

Given the distinctive characteristic of the fourth generation of revolutionary theory as a self‐professed project— an “agenda to be fulfilled” rather than an historical interpretation (Abrams, 2019; Lawson, 2016, p. 110)— discarding it as the latter would be a category error with pernicious consequences.


Despite its initial promise, the influence which the fourth generation project exerted on the field of revolutionary theory eventually became – for numerous reasons (see: Abrams, 2019; Allinson, 2019; Beck, 2018; Lawson, 2016; Ritter, 2019) – a limiting one.

There are lessons to be learned from the fixation of a whole body of scholars on conjunctural modelling and ontological discourses intertwined with the progressive decrease in scholarly rigour observed by analyses of work in the field (Beck, 2017).

Worse still, as Beck (forthcoming) has noted, it seems that “what we think we already know about revolution may very well be probably wrong.” Amid all the problems we have encountered over the years, should we decline to consciously break with the conditions which cultivated them we risk doing more to preserve our field's current impasse than to move past it.

We risk allowing the much decried maladies of the fourth generation project to become embedded in the complex web of scholarship that constitutes work on the subject of revolution.

It is for this reason I would dissent from the suggestion that that the future of revolutionary theory demands that we “free ourselves from wondering what the fourth generation was or is, and what the fifth generation is or will be,” (Beck & Ritter, 2021) Rather, reflection on where progress has tended to stall and how we might do better is vital to the renewal of the field.

Refusing to acknowledge the fourth generation project's initial promise, latter faltering, and eventual decline not only obscures the great deal of intellectual effort poured into its ultimately unsuccessful project, but also renders critical reflection on its scholarly agenda all the more difficult.

This, in turn, might allow certain extant ideas associated with the fourth generation to remain as hegemonic than they have been in the past, yet also insulate them from critique.

All this might leave us “profoundly in the thrall” of past perspectives – as Eric Selbin (1999, p. 10) once decried of fourth generation theories.


Elsewhere, Beck (forthcoming) has rightly noted that “thinking of prior theory in generational terms has overstated the extent of consensus present at any time.

” The same accusation can be made against Ritter and Beck's claim about the extent of consensus present in our field today.

While Ritter and Beck seem content to break with the practice of generational thinking, they nonetheless appear to present some of its symptoms by depicting as ‘consensus' what are in fact still quite clearly matters of scholarly difference.

This arises with respect to two crucial areas of revolutionary theory: the object of study, and method of inquiry.

Truly thinking beyond generations necessitates that we reject the notion that we have settled certain major debates in revolutionary theory and now need only to perfect our knowledge of revolution. Should we rid ourselves of generational language, but retain the biases of generational thinking – that certain matters are now off the table, and that our field has a single renewed purpose –we risk being trapped ‘beyond and within’ the fourth generation of revolutionary theory, having formally eschewed its aesthetic trappings, but unable to escape its more substantive shortcomings.


It is tempting to think that theorists of revolution generally agree on what exactly it is that they are studying.

In line with such a view, Ritter and Beck seek to claim that revolutionary theory's “object of study is clear to contemporary scholars.

” They argue that revolutionary theorists are united in studying “attempted and successful shifts in, at the very least, regimes or political structures that occur through mass mobilization.

” This is far from a consensual or established definition.

Not only would it exclude, for example, much of the history of the French Revolutionary period (a classic case in our field), but also call into question the study of central questions in revolutionary theory such as ideology, culture, micro‐mobilization and elite agency.

This is not even to mention currently popular areas of focus such as revolutionary preconditions, regime collapse, revolutionary and counterrevolutionary outcomes (Allinson, 2019).

Conversely, such a definition of revolution might puzzlingly include cases like the Obama‐era Tea Party movement in the United States, which utilized mass mobilization in order to shift political structures (and arguably eventually the governing regime).

The US Civil Rights Movement also substantially shifted political structures through mass mobilization, and – depending on how one defines a regime ‐ one could even imagine a grassroots election campaign constituting an appropriate object of study as per Ritter and Beck's definition of revolution.

There is a substantial variety of definitions of revolution that differ widely from the one Ritter and Beck argue is ‘clear’ to contemporary scholars.

One such scholar, George Lawson (2019:10), considers a revolution to constitute “a collective mobilization that attempts to quickly and forcibly overthrow an existing regime in order to transform political, economic, and symbolic relations.

” This definition shifts the onus from mass mobilization to merely collective ones, but also introduces new criteria such as speed and force, as well as economic and symbolic transformations.

By contrast, Selbin (2008:131) considers revolution to be a “conscious effort by a broad based, popularly mobilized group of actors, formal or informal, to profoundly transform the social, political, and economic institutions which dominate their lives; the goal is the fundamental transformation of the material and ideological conditions of their everyday lives.

” This definition introduces ideas of intention, the source and breadth of mobilization, and the extensive transformation of various institutions as well as of material and economic conditions.

Yet other scholars – such as John Dunn (2018) – have even contended that revolutions no longer occur, arguing that they are defined by a transformative ideology that present day cases lack.

This line of contention has led to substantial recent disagreement over whether to define revolution as a political process at all, or whether a programmatic definition is preferable (Abrams, 2018; Abrams & Dunn, 2017; Ritter, 2019; Slim, 2018).

Such is the degree of difference over revolution as an object of study that Ritter and Beck's proposed consensus definition also breaks with ones they each have offered very recently.

Ritter (2019) has recently defined revolutions as “simply an irregular overthrow of a political regime through mass mobilization.

” Ritter thus excludes ‘political structures’ and ‘attempts’ from his definition.

Beck (2016:17) has previously defined revolutions as “mass contention against an existing state [which] overthrows a regime, or creates lasting social change and establishes new political structures.

” This is also a narrower object of study than the pair's joint definition: it specifies revolution as a state‐centric phenomenon, and specifies that – in the absence of regime overthrow ‐ durable social change should occur and novel political structures should be created rather than only shifted.

For my own part, I consider a revolution to constitute a popular overthrow of the power structures in an existing polity.

These can of course be stipulated as successful, failed, ongoing, or even imagined.

In some regards this definition is stricter than Ritter and Beck's: the US Civil Rights Movement – which notably altered, but did not wholly overthrow the power structure in the United States – would not be counted as a revolution.

In other regards, this is a much more expansive definition: it unchains revolution from the state or regime, and includes in its scope the popular overthrow of power structures on a small scale (e.g. a workplace, locality, or institution) and a very large scale (e.g. the international order).

All this is suffice to say that our object of study is still subject to a plurality of interpretations.


Just as it is tempting to think that revolutionary theory no longer harbours disagreement about its object of study, it is equally attractive to think that “debates over method are… of a time that is passed.” ( Beck & Ritter, 2021).

Beck and Ritter argue that the solution to these debates is to be found in “a pluralistic methodology” that tends to (though might not always) combine quantitative and qualitative methods for optimal results.

Such a line of contention was prominently advanced in Goldstone's Toward a Fourth Generation of Revolutionary Theory (2001:175), which concluded that fourth generation studies would “unify the results” of a variety of qualitative and quantitative methods.

Just as Ritter and Beck contend that “we should not expect one subfield to solve the structure‐agency problematic,” it is similarly unrealistic to suggest that revolutionary theory has solved the methodological one.

Indeed, Beck (2017:533) himself has observed “a lack of consensus… and little convergence over time,” over issues such as case selection and research strategy.

Elsewhere, he has remarked that the study of revolution exhibits “no convergence on the study design techniques that comparative methodologists recommend.

” ( Beck, forthcoming).

The mixed method studies to which Ritter and Beck assign preference are not necessarily superior to single‐method modes of inquiry.

Indeed, though they sometimes offer substantial benefits, mixed method approaches are at other times inappropriate, impractical, illogical, or unnecessary for attacking some of the most interesting research puzzles which we come across (Halcolmb, 2018; Loo & Alan, 2011; Symonds & Gorard, 2010; Timans, et al., 2019).

Though analyses which bridge quantitative and qualitative approaches may indeed sometimes yield “the advantage of both,” there are also occasions in which a slapdash historical survey or a surface‐level suite of descriptive statistics does little to enrich scholarly analysis, and yet efforts to produce them might still take up vital portions of an article's word count, or cut‐short more enriching research pursuits.

Conversely, there are also many cases in which profitable mixing of methods occurs strictly within either the qualitative or quantitative methodological silos.

On still other occasions, there are times where single‐method studies yield new empirical or theoretical discoveries that were attained precisely because of the practitioner's commitment to honing and furthering their chosen method.

Given the surprising methodological poverty in the study of revolution (Beck, forthcoming) it is particularly important that we should prioritise that methods be well executed, rather than that they be combined.

There are many opportunities available to revolutionary theory should we harness a greater number of methods than we have in the past, but this is not simply achieved by harnessing past methods simultaneously.

Instead, we should be encouraging a greater variety of methods – be they singular or mixed– in order to further our understanding of revolution's multiple dimensions.

Sometimes, this will take the form of Ritter and Beck's proffered “pluralistic methodology,” but we must not allow excitement about such approaches to obstruct a broader pluralism about methodology.

Only by paying attention to scholars with diverse and disruptive methods, as well as experts in established methods and mixed‐methods analysts will we open the field for new and challenging discoveries.


Beck and Ritter are right to argue that we must pay careful attention to the “continuity of debates over the decades.” However, we cannot simply regard these debates as settled merely because scholars no longer disagree as violently as we imagine they might have in the past.

In my view, it is not so much that past debates have been settled in favour of a clear or consensual answer – as Beck and Ritter contend ‐ but rather that other scholarly fashions rose to prominence.

There are persisting differences regarding our object of study or how we might study it, and the empirical variety of revolutions and ways to study them have only increased.

It is precisely by means of this increase in cases and studies that we have seen that there is a great deal more to talk about than the hypothetical perfection of definitions or methods.

While there are many compelling reasons to move past the classification of revolutionary theory into neatly organized ‘generations,’ a conscious and critical acknowledgement of the fourth generation project is nonetheless necessary.

This is because of its status as a specific intellectual project, and its prominent role in many scholars' theoretical education.

This role has not always been productive.

As Ritter and Beck astutely note, the ‘generational thinking’

which accompanied the fourth generation's rise to scholarly prominence obscured valuable work, made invisible certain processes of knowledge accumulation, and is at this point limiting our theoretical imagination.

Just as they note that this mode of thinking cultivated a “scholarly fear” of returning to past insights, we should not exhibit that same fear with regard to revisiting past debates.

Beck and Ritter are right to critique the generational framework used to chart the history of revolutionary theory, but dispensing with this notion does not mean that the fourth generation project should be given a free pass.

When Beck and Ritter condemn generational thinking as a false premise, they condemn the foundational premise of the fourth generation project as equally false.

Nowhere has the generational framework been more celebrated than in the work of self‐professed fourth generation scholars, and the generational teleology of revolutionary theory is arguably more vital to ‘Fourth Generation’ accounts than any other attribute.

It is precisely because fourth generation theory served as a backdrop for the obfuscation of scholarly history – rightly bemoaned by Ritter and Beck‐ that we should welcome a critical evaluation of its ideas, and open discussion of where we might go next.


Ask any rising scholar in the study of revolution, and they will agree that our field finds itself in a pivotal moment.

Allinson (2019) has argued that existing theories of revolution are failing to keep pace with empirical developments.

Ritter (2019:106) and Beck (forthcoming) have gone even further, declaring the death and rebirth of our past approaches to studying revolution: “The Revolution is dead, but long live the revolution,” as Ritter puts it; “Revolution studies is dead; long live revolution studies,” as Beck does.

But once we break with the thinking of the past and its generational imagery, what – as the old revolutionary question goes – is to be done? How might we reopen our minds after decades of generational thinking and actively work to regenerate revolutionary theory? With regard to our hopes for the future of revolutionary theory, there is much that Allinson, Beck, Ritter and I have in common.

All of us call for a break with the past.

Whether we call what comes next a new ‘generation’ or simply “a new future for the study of revolution,” as preferred by Ritter and Beck, we all hope that it will be characterised by an influx of novelty, be it new theories, cases, phenomena, or interpretations.

There are still more valuable sources of novelty in addition to those imagined by Ritter and Beck.

We might also – for example ‐ hope to see the arrival of novel research methods, ethical standpoints and practical tools.

We might seek dialogue with new fields or reinvigorate our relationships with those we now tend to ignore.

Regenerating revolutionary theory will require more than novelty alone.

It is not enough to simply resume existing debates on the basis of uncritically inherited premises.

We can and must go one step further and reopen old conversations, question received knowledge, and initiate new debates altogether.

This means that we should not exclusively align the advancement of our field within the constraints of a rigid and immovable object of study, nor amid the strictures of methodological orthodoxy.

Instead, we should make room for disruptive methods, challenging epistemological positions, dialogue with practitioners, and research from new or unexpected sources.

Casting aside generational thinking (and with it the strictures of the fourth generation project) may well be fruitless if we do not think carefully about where our field has gone astray, and how we might pursue more constructive directions.

We risk the degeneration of revolutionary theory into either a false narrative of consensus or isolated siloes of study with disconnected debates and literatures.

Thus, in the years to come scholars should consciously pursue the deliberate regeneration of our field: building connections between debates and fields, educating new scholars, casting aside honorific hierarchies, and celebrating disagreement and diversity.

If we are to advance the field of revolutionary theory we need to explore in multiple directions, lest we find ourselves on yet another winding path or risk making a wrong turn.

Only an open field of study can provide insurance against such a fate.

To join in what is a fast emerging scholarly trend: the fourth generation of revolutionary theory is dead, long live the regeneration of revolutionary theory.


The incidence of type 1 diabetes in many developed countries has been increasing at rates faster than can be explained by the known genetic propensity towards the disease [1].

The envi- ronmental triggers of this disease have not yet been identified, despite the many efforts to associate diet, vitamin D, viruses and other factors with disease [1].

A leaky gut has been cor- related with type 1 diabetes [2], and an aberrant gut microbiome was proposed as the factor that results in a leaky gut followed by altered immune responses leading to disease [3].

This review describes a model for the role of the gut microbiome in type 1 diabetes based on the latest results.


Past work with non-obese diabetic (NOD) mice and BioBreeding diabetes-prone (BB-DP) rats provided the first Microbiology and Cell Science Department, Institute of Food and Agricultural Sciences, 1355 Museum Road, PO Box 110700, Gainesville, FL 32611-0700, USA Diabetologia (2015) 58:1386-1393 DOI 10.1007/s00125-015-3614-8 evidence that bacteria may play an important role in the onset of type 1 diabetes.

BB-DP rats fed sulfamethoxazole, trimeth- oprim and colistin sulphate demonstrate a significantly de- creased incidence of diabetes compared with controls [4].

A similar increase in disease-free animals vs controls was ob- served in NOD mice after administration of doxycycline or vancomycin [5, 6].

Bacteroides were higher in stools from the BB-DP rat while Lactobacillus was higher in the diabetes- resistant rat (BB-DR) [7].

Strains of Lactobacillus johnsonii and Lactobacillus reuteri isolated from BB-DP rats prevented and promoted diabetes in BB-DP rats, respectively [8].

The L. johnsonii strain induces a T helper 17 cell bias in the mes- enteric lymph nodes of C57BL/6 mice while the L. reuteri strain does not [9].


Although the incidence of insulitis is not significantly dif- ferent between germ-free and specific-pathogen-free (SPF) NOD mice, germ-free mice develop insulitis earlier than SPF mice [10, 11].

The presence of segmented filamen- tous bacteria (SFB) in female NOD mice is correlated with decreased insulitis incidence and with the percent- age of IL-17-expressing lymphocytes [12].


Diet is known to significantly affect diabetes incidence in NOD mice.

For example, a high-cereal diet increases diabetes incidence in NOD mice, while a high-protein diet reduces risk [13].

Recent studies observed that acidic drinking water also increases diabetes incidence in NOD mice, and this is prevented by inoculating mice with SFB [14].


The results of the murine experiments encouraged efforts to determine whether associations between gut bacteria and type 1 diabetes could be discovered in humans.

Rodent model in- vestigations suggested that healthy children may have high populations of probiotic-like bacteria such as Lactobacillus, while the gut microbiome of unhealthy children may be dom- inated by Bacteroides.

Perhaps antibiotic or probiotic use ear- ly in life could prevent type 1 diabetes, but this requires more study.

Analysis of a Finnish dataset revealed no connection between antibiotic use and autoimmunity for type 1 diabetes [15], but larger cohorts may be needed to observe an antibiotic influence on this disease.

Early on, a simple picture was ex- pected to emerge by simply studying the stool content of a few human cohorts with individuals at high genetic risk of type 1 diabetes.


At first, with very small human cohorts with only four healthy children and four children with type 1 diabetes auto- antibodies from the Diabetes Prediction and Prevention (DIPP) study in Finland, the human situation did appear to be very similar to the murine one [16, 17].

These studies showed highly significant taxonomic and functional differ- ences between cases and controls prior to autoimmunity for type 1 diabetes.

Levels of Bacteroides were higher in children with autoimmunity, and levels of seemingly protective unclas- sified Firmicutes were higher in healthy children.

These re- sults match the findings of the murine experiments except for the fact that the human samples had negligible amounts of Lactobacillus.


Contrasting results were obtained from an investigation of 298 stool samples collected during the first 3 years of age from 22 case and 22 control children enrolled in the German BABYDIET study [18].

Unlike the small Finnish cohort, no significant differences between taxa were observed between cases and controls after correction for false discovery rate.

Instead, network analysis showed that bacterial communities in cases were far less strongly correlated with each other at <6 months of age and again at about 2 years of age than in controls.

However, the results from the Finnish and German cohorts were similar in that the autoimmune bacterial commu- nities were less stable than the healthy communities.

The cause for this instability in autoimmune samples remains unknown.


A still larger set of 947 stool samples collected during the DIPP study from 29 cases and 47 controls during the first 4 years of life was examined [15].

All of the children enrolled in the DIPP study are genetically at high risk for type 1 dia- betes.

Analysis of these samples revealed a very striking re- sult: approximately 8 months prior to the average time of appearance of the first autoantibody in cases, a large increase in one bacterial species, Bacteroides dorei, was observed.

In addition, B. dorei was by far the most abundant species in autoimmune prone children, with over 20% of the population represented by this one species-a rate that was more than double the relative abundance of this species in healthy chil- dren.

This difference was highly statistically significant even after correcting for false discovery rate and was found to occur at about the same time as the introduction of solid food.

A few other minor taxonomic differences were also observed, but nothing as striking as the B. dorei result.


So why was the B. dorei result seen in the large Finnish DIPP cohort but not in the German BABYDIET study? At first glance, it may appear that since the B. dorei result in Finland was not reproduced in Germany, it must then be an anomaly.

However, there is one very important difference be- tween the designs of these two cohorts that may explain this difference.

The samples used in the DIPP study were all from children born in the same hospital, Turku University Hospital [15].

All of the children lived within 80 km of each other.

Thus, many of the known confounders of the microbiome community were controlled; that is, climate, diet, culture, wa- ter supply, pollution levels, air quality and medical practices were all very similar for these children.


In contrast, the children in the BABYDIET cohort were from all over Germany, allowing enough variability in the microbiome confounders to affect microbiome composition.

In addition, all of the children in the BABYDIET study had Diabetologia (2015) 58:1386-1393 1387 first-degree relatives with type 1 diabetes, while in DIPP the children were chosen for the study based on their high-risk HLA genotype at birth.

These confounders add to the noise of the data and may explain why no significant taxonomic dif- ferences were observed between cases and controls.

These confounders may have been sufficient to mask any taxonomic differences that might otherwise have been detected in the BABYDIET study but too weak to mask the network differ- ences seen in this cohort [18].


Hence, a working hypothesis is that geography plays a very large role in whether it is possible to observe significant case- control differences in any cohort designed to examine associ- ations between the microbiome and disease.

From this per- spective, the microbiome results from the BABYDIET and DIPP cohorts do not contradict each other but rather help us understand how best to design cohorts for future disease- related microbiome studies.


Two other recent studies from Europe and Mexico showed taxonomic differences in the gut microbiome between individ- uals with type 1 diabetes and individuals without the disease [19, 20].

In both studies, levels of Bacteroides were signifi- cantly higher in those children with type 1 diabetes compared with healthy children.

These studies were intended to show the effect of the disease itself as opposed to identifying a trigger for autoimmunity as in the previous studies.

The Mexican study was restricted to Sonora state, which borders Arizona, and was limited to 29 children, including eight con- trols.

Based on the findings of these studies and the pre- autoimmunity cohorts described above, Bacteroides appears to be a major contributor to microbiome dysbiosis both prior to the development of autoimmunity and after disease diagnosis.


There is evidence that the human genome has some control over the taxonomic composition of bacteria in the gut.

However, it is not known whether a genetic propensity to type 1 diabetes, as manifested by HLA genotype, affects bacterial gut composition [21].

The studies conducted to date on the relationship between type 1 diabetes autoimmunity and the microbiome have not seen an HLA genotype effect on the microbiome, but these studies did not have enough participants to observe such an effect [15, 18].


The hygiene hypothesis suggests that a lack of expo- sure to microbes early in life under hygiene conditions in the developed world contributes to a weakened im- mune system incapable of warding off the effects of detrimental bacteria in the gut [22].

Efforts are under- way to examine this issue carefully between Finland and neighbouring Estonia and Russian Karelia [22].

Testing this hypothesis will be difficult in any study protocol.

Our view that separating signal from noise is best done by careful cohort design intended to reduce the confounders of the microbiome.


Our recent results suggest that striking case-control differ- ences can be observed when the geographic distribution of a high-risk cohort is limited to one city and its immediate sur- roundings.

Another issue is whether type 1 diabetes risk is higher in urban or rural areas [23].

Differences in large gut microbiome taxa and alpha diversity differences were recently observed across six clinical sites in children at high genetic risk for type 1 diabetes [24].

Higher socioeconomic status is typically associated with type 1 diabetes and other autoim- mune diseases [25].

To date there are no reports in the litera- ture on socioeconomic status and gut microbiome composi- tion, but socioeconomic status has been shown to be associat- ed with the oral microbiome [26].

However, given that geog- raphy and diet are well known to affect gut microbiome com- position and that socioeconomic status affects diet and place of residence, cohort design should take these factors into ac- count or in some way control for them.

To date, no clear associations between type 1 diabetes and diet have been made, but this is being carefully examined by The Environmental Determinants of Diabetes in the Young (TEDDY) study group [27].


An ideal cohort design might be one that includes samples from a set of cities similar to Turku where the cohort includes children all born in the same hospital and all living in the surrounding area.

The variability of other environmental fac- tors, such as climate, drinking water, health practices and air quality, are similar across a single city.

Having a set of such cities each with children at high risk might allow us to identify bacteria or bacterial functions that are associated with autoim- munity for type 1 diabetes across a broad geographic scale.


Sampling from families may be a means of reducing the noise in the microbiome data.

That is, rather than collecting samples from healthy controls matched for genetic risk, con- trol samples could be obtained from healthy siblings.

But age matching is important, so noise could be further controlled by enrolling identical and non-identical twins for the study of those at high genetic risk for the disease at birth.

This would be a difficult undertaking in a small city such as Turku but might work well in a much larger city in the same country, such as Helsinki.


Another way of reducing noise would be to determine the changes that can occur in the gut microbiome communities in stools in response to long-term exposure to room temperature and air.

Approaches must be developed that make it easier for families to collect stools in a manner that allows for rapid freezing.

Microbial communities in stools do change signifi- cantly after 24 h of exposure to air, and these changes include a sharp decline in the relative abundance of Bacteroides strains [28], which can survive air but do not divide under aerobic conditions.

To date, none of the studies to investigate 1388 Diabetologia (2015) 58:1386-1393 the microbiome of children at high genetic risk for type 1 diabetes has been designed with the microbiome in mind.

In addition to the aforementioned improvements in study design and larger cohorts with limited confounding, advancements in big data analysis methods should also help to facilitate within- and between-study analyses.


As described above, a recent study showed a strong associa- tion between high levels of B. dorei in the gut several months prior to the appearance of the first type 1 diabetes autoanti- body [15].

A first step towards determining whether B. dorei causes type 1 diabetes autoimmunity would be to culture it from the Turku DIPP samples, then inoculate NOD mice with this organism and determine whether this strain(s) reduces the survival rates of the mice.

Although this is a high-risk strategy and not without significant intellectual challenges, it would be worthwhile to conduct such an experiment because even though B. dorei may not be involved in type 1 diabetes onset in other populations, a certain set of functions expressed by B. dorei may have wide applicability to autoimmune disease around the globe.


As more bacterial metagenomes are obtained from stool samples of high-risk children, quantifying the relative abun- dance of homologues or paralogues of B. dorei genes will be an easy task.

Additional Bacteroides species may be associat- ed with type 1 diabetes autoimmunity in other cohorts, which will allow us to build a database of genes in common with all Bacteroides associated with diseases vs those genes in Bacteroides species not associated with disease.

A similar approach can be used on bacterial species or strains thought to be protective against the disease.

Bacterial functions asso- ciated with disease have been listed for a very small cohort from the DIPP study [16].

Further studies are warranted on an increased number of individuals and more samples across ad- ditional time points.


As Bacteroides have been associated with type 1 diabetes autoantibodies, the next obvious question pertains to the source of these bacteria.

That is, if a single species or set of closely related species is associated with type 1 diabetes, the need to hunt for more bacterial associations with type 1 dia- betes declines and the problem becomes one of searching for those environmental factors that increase the abundance of these Bacteroides species in the gut.

In the stool samples of autoimmune Turku children, levels of B. dorei peak at about 7 months of age, about 1 month prior to the Finnish national standard for the timeframe of solid food introduction [15].

If the early introduction of solid food is related to raised Bacteroides levels, which foods are known to increase Bacteroides abundance in the gut?


High-fat and high-gluten diets encourage colonisation of the gut by Bacteroides.

Bacteroides populations in the human gut have been strongly correlated with a diet high in protein, animal fat, cholesterol and other fats [29].

Bacteroides are well known as the primary proteolytic bacteria in the gut [30].

Not surprisingly, vegans and vegetarians have low levels of Bacteroides in their stools [31].

To date, connections between a vegetarian diet and the progression towards type 1 diabetes have not been studied.


Examination of the gut microbiome in coeliac disease sug- gests that gluten may play a role in Bacteroides abundance.

The prevalence of B. dorei was 67% higher in children with active coeliac disease compared with the same children on a gluten-free diet for two years [32].

Similarly, formula-fed chil- dren at high risk for coeliac disease were approximately six- times more likely to possess B. dorei in stool than low-risk controls [33].

These results encourage further work to deter- mine whether gluten increases population growth of B. dorei in children at high risk for autoimmunity.

Resistant starch may also play a significant role in preventing type 1 diabetes as it has been reported to increase faecal butyrate levels [34].


The onset of type 1 diabetes might involve an aberrant intes- tinal microbiome creating the conditions for gut leakiness followed by mucosal immune responses that lead to an attack of insulin-producing islet cells [3].

Consistent with this notion, type 1 diabetic individuals have been shown to have a leakier gut than controls [35-37], and it has been reported that this leakiness is not caused by type 1 diabetes, rather, type 1 dia- betes is preceded by a leaky gut [2].

In the NOD mouse model, a leaky gut can lead to the activation of diabetogenic CD8 + cells which, in turn, lead to the development of insulitis [38].

Mechanisms for a connection between the leaky gut and type 1 diabetes have been reviewed previously [39, 40].


Butyrate is one of the short-chain fatty acids (SCFAs) pro- duced by bacterial fermentation in the gut.

The ratio of butyrate:acetate:proprionate in the gut varies widely, but these three SCFAs make up the majority of the SCFA content in this organ.

The concentration of SCFAs in the gut can be as high as Diabetologia (2015) 58:1386-1393 1389 140 mmol/l and these SCFAs can provide up to 10% of the daily human caloric needs [41].

Butyrate reduces trans- membrane transport of Escherichia coli across confluent monolayers of colon-derived cell lines [42].

In addition, in- flammation may reduce butyrate transport through the down- regulation of a butyrate transporter [43].


Butyrate is also known to play two important roles in maintaining the integrity of the epithelial layer.

First, butyrate induces the expression of mucin-producing genes in the epithelial layer [44].

Mucin is a host gly- coprotein that protects the epithelial layer from attack by bacteria or bacterial metabolites.

Second, butyrate restores and maintains tight junction assembly in epithe- lial layers [45, 46].

These tight junctions are formed between epithelial cells and serve as gatekeepers for metabolite transport across the epithelium.

Other SCFAs do not induce tight junction formation or mucin synthesis.


Healthy children have a higher proportion of butyrate- producing gut bacteria in their microbiomes compared with children expressing at least one beta islet cell autoantibody [16, 47].

Based on this, the ratio of butyrate-producing to other SCFA-producing bacteria in the gut was proposed to be a key regulator in determining the gut health of a child at high risk for type 1 diabetes [16].

However, there is no evidence to date linking butyrate levels in the gut to the progression of diabetes in either animals or humans.


Butyrate has roles other than maintenance of the integrity of the gut epithelial layer.

In particular, there is increasing evi- dence that butyrate has epigenetic effects that may be very important in type 1 diabetes, as unknown environmental trig- gers must be playing a role in the rapid increase in the inci- dence of this disease [48].


Butyrate induces the methylation of promoter regions, which causes both up- and downregulation in different sets of human genes [49].

Histone acetylation also appears to be regulated by butyrate production.

Butyrate reduces lipopolysaccharide-induced inflammation in the intestine through modulation of antioxidant defence systems [50], nitric oxide production, and expression of inflammatory cytokines [51].

Many of these effects appear to be modulated by the inhibition of histone deacetylases in macrophages [51].

Also, histone acetylation in the promoter of the Foxp3 locus Beginning with the introduction of solid food, consume a diet predominant in:


The genomes of two of the B. dorei strains from the DIPP Turku cohort have been sequenced to completion and, physi- ologically, appear to be typical Bacteroides in that they metabolise a broad spectrum of compounds and have many antibiotic-resistance genes [54].

One of these genomes was derived from a child who later developed type 1 diabetes- associated autoantibodies (case) while the other genome was from a high-risk healthy child (control).

The case genome in- cludes a bacteriophage with a DNA adenine methyltransferase gene that appears to be responsible for the methylation of all but three of the 20,554 GATC motifs in the genome [54].

Such phage-borne DNA methyltransferases have been found in sev- eral bacteria [55].

None of the nearly 19,000 GATC motifs in the control genome were methylated, presumably because it lacked this phage and its orphan DNA methyltransferase gene.

This work was the first report of DNA methylation differences within a bacterial species in the microbiome [54].


Although these methylation results from two B. dorei ge- nomes cannot be associated with disease, they suggest that a wider examination of methylation patterns in more samples is warranted.

DNA methylation patterns are known to affect gene expression in bacteria [56].

They control epigenetic re- sponses that are often regulated by environmental exposure.

Indeed, a few methylation sites in the human genome have been associated with type 1 diabetes pathogenesis [57, 58].

It is therefore conceivable that methylation differences in the genomes of bacteria associated with type 1 diabetes may play a role in disease.


The analyses described above suggest a revised model for the role of the microbiome in type 1 diabetes, one that is based on a previous model [16] but takes into account the role of diet (Fig. 1).

In this revised model, type 1 diabetes autoimmunity occurs when a high-fat, high-gluten diet, perhaps one with a low level of resistant starch, increases Bacteroides colonisa- tion in the gut.

These bacteria ferment glucose to SCFAs other than butyrate.

The lack of butyrate production reduces the likelihood of an intact gut epithelial layer.

The resulting per- meability causes bacterial antigen exposure that induces im- mune responses leading to type 1 diabetes autoimmunity.


Recent results suggest that the many confounders of gut microbiome composition need to be considered in cohort de- sign for the optimal discovery of associations between the gut microbiome and disease.

A cohort carefully designed to con- trol for geography and diet as much as possible can eliminate much of the noise that is often associated with microbiome data.

Future work will require experiments or observational studies that directly test each point in the model (Fig. 1).


Environmental factors play a role in the development of type 1 diabetes.

There is growing enthusiasm towards assessing the role of gut microbiota as an environmental modifier of autoimmunity in type 1 diabetes.

This is largely based on the increasing awareness of the role of microbiota in regulating gut homeostasis and immune responses in the gut [1].

Microbiota composition varies between individuals and is modified by diet [2].

An emerging number of studies suggest that the abundance of Bacteroidetes is associated with autoimmunity in type 1 diabetes [3].

Thus, a diet promoting an abundance of Bacteroidetes could be an environmental risk modifier in type 1 diabetes.


The notion of Bacteroides as ‘diabetogenic’ species is now further supported by a recent study in which Bacteroides dorei was found to be abundant in children who subsequently developed diabetes-related autoantibodies in a cohort of 76 children participating in the Diabetes Intervention and Prevention Project (DIPP) study in Southwest Finland [4].

In NOD mice, diabetes incidence is effectively modified by diet [5].

We have studied the effect of two common nutritional fibres, pectin and xylan, on microbial colonisation, gut homeostasis and diabetes development in NOD mice [6].

Pectin and xylan dramatically increased colonisation of the intestine by Bacteroidetes, including B. dorei (Fig. 1 and data in [6]), increased the expression of stress-related molecular transcripts and inflammatory cytokines in the gut, and altered the balance of T cell cytokines in pancreatic lymph nodes.

In terms of disease-promoting mechanisms, our findings alluded to a pathophysiological model [6] well aligned with the model recently discussed by Davis-Richardson and Triplett in Diabetologia [3].

The core idea is that, by displacing beneficial butyrate-producing taxa, Bacteroidetes have adverse effects on epithelial cells [3, 6].

Our findings provided experimental evidence in support of this and indicated that at least some Bacteroides species play a role in diabetes-related autoimmunity.

In addition, our results identified dietary fibres as novel candidates for environmental modifiers of the pathogenesis of diabetes.


Interestingly, induction of low-grade inflammation in the gut and microbiota encroachment was recently reported in mice following administration of two other fibre compounds used as dietary emulsifiers [7].

According to Finnish healthcare recommendations, fermentable fibres including pectin and xylan (hemicellulose, a constituent of all plant cell walls) are introduced into the diet of infants in the form of berry cocktails.

In Finland, parents are recommended to start giving berry cocktails to their infants at 4 months age, an age preceding the peak in the observed abundance of B. dorei [3].


Bacteroidetes are capable of adapting to different nutritional environments and they possess a large number of genes that encode carbohydrate-active enzymes.

After weaning, in the murine intestine, Bacteroidetes start to express a plethora of enzymes that participate in the liberation of simple sugars from complex polysaccharides of dietary fibres [8].

Simultaneously, the expression of genetic loci encoding enzymes involved in the synthesis of capsular polysaccharides is modified [8].

The immune-stimulating and metabolic effects of different members of the microbiota are still incompletely understood, and differences are likely between different individuals’ bacteria representing even the same species, including B. dorei [3].

We envision that fermentable fibres may modify the function and immunogenicity of microbiota, including members of the Bacteroidetes phylum, in ways which may have relevance to type 1 diabetes [6].


To the Editor: We agree wholeheartedly with Hänninen and Toivonen [1] that we must move beyond taxonomic associations of bacteria with type 1 diabetes.

However, a strong bacterial taxonomic association with disease, such as with Bacteroides dorei and type 1 diabetes autoimmunity in Turku, Finland [2], can provide a considerable number of clues about how the functions of this bacterium can lead to disease.

When a bacterial species is known at the genus and species levels and associated with disease, this allows us to make many predictions about how this organism may be dominating a particular environment.

In the case of B. dorei and type 1 diabetes in Finland, this has gone further, with the publication of three full genomes of this species from stool samples obtained in Turku.

This work led to the model described in our recent review [3].

The model was intended to serve as a platform to test the ideas presented, as well as to add new concepts.

That is, if B. dorei is associated with type 1 diabetes autoimmunity, what are the factors that contribute to its dominance in the gut microbiome of unhealthy children?


As stated in the review, a number of papers have shown that diet can affect Bacteroides populations in the human gut.

The authors propose the addition to fibre to our model for the dietary control of Bacteroides gut populations [1].

This is based on their recent study in the NOD mouse [4], the idea being that fibre in the diet may be an important component leading to type 1 diabetes and that this may be conferred by an increase in the Bacteroides population in the NOD mouse.

It is not clear whether this work in mice is translatable to the human condition.

The authors propose that a fibre-containing berry cocktail given to Finnish children at 4 months of age may be the trigger that precedes the increase in B. dorei at about 7.5 months of age.

Solid food is introduced to Finnish babies at 4–6 months of age [5, 6], so it is hard to identify the particular dietary components that contribute to a high B. dorei population, but we agree that soluble fibre is one candidate.


As stated previously [2], it has been known for many years that Bacteroides likely plays an important role in the digestion of complex carbohydrates in the human gut [7].

Nevertheless, there are precious few papers on the role of dietary fibre on Bacteroides populations in the human gut.

A recent paper by Lamichhane et al [8] reported a strong positive correlation between soluble fibre intake and the level of Bacteroides in the human gut.

Still other papers point to gluten as a contributor to a high relative proportion of Bacteroides species [9], while others suggest that high protein and animal fat is an important contributor [10].

It would not be surprising that all of these dietary factors play a role.

However, it is clear that the source of B. dorei in Turku, Finland, will never be resolved by studies in the NOD mouse, and a careful epidemiological analysis of the source of this bacterium in that city is warranted.


Decentralized wastewater treatment has been proposed as a strategy to help reduce potable water consumption (e.g., Wilderer and Schreff, 2000) and as a cost-efficient alternative to more centralized treatment (e.g., Engin and Demir, 2006).


Decentralized treatment systems are defined as the collection, treatment, and distribution of water and wastewater near the point of use or generation (Crites, 1998).

These decentralized systems reduce collection transport distances and have lower flow volumes that enable the use of smaller diameter piping, shallow installation depths, and vacuum and pressurized sewers (Nelson, 2005), all of which potentially reduce energy and material use.

Complementing centralized water-related infrastructure with decentralized facilities has been described as a "soft path solution" (Gleick, 2003), partly because decentralized facilities allow water services and quality to be tailored to end-use needs.


Decentralized systems are also more amenable to grey water separation technologies (Lienert and Larsen, 2006), which increase the potential reuse of wastewater and promote the return of treated wastewater within the watershed of origin (Massouda et al., 2009).

Decentralized wastewater systems can be located adjacent to areas with high demands for non-potable water, such as golf courses and public landscaping, thereby redirecting large volumes of water for reuse (Allen and Vonghia, 2005).


The significant capital investments required for centralized treatment infrastructure can be prohibitively high in developing countries (Rocky Mountain Institute, 2004) and decentralized wastewater systems are perceived as a more appropriate strategy with reduced costs, less maintenance, greater flexibility in planning for future growth, and reduced environmental impact (Engin and Demir, 2006).

For these reasons, institutions like the World Bank consider decentralized systems as an alternative to traditional centralized systems (Wilderer and Schreff, 2000).


While there are many potential benefits of decentralized systems, the inherent loss of scale economies relative to larger centralized systems may result in increased unit energy, cost, and materials associated with facility operation as treatment plant capacity decreases.

These benefits of centralized systems, however, are thought to diminish when building and maintaining the distribution and collection systems are considered (Lens et al., 2001).

A proper environmental analysis and comparison of centralized and decentralized wastewater systems requires expanding the evaluation scope beyond facility operation to determine the impact at each stage of the process, including infrastructure construction and maintenance as well as material production and transport.

This paper provides a framework for analyzing decentralized wastewater systems and evaluates the energy and environmental impacts associated with two currently operating case systems - one centralized and one decentralized - while accounting for the resources consumed and the pollutants released throughout the life-cycle of the collection, treatment, and distribution processes, as well as accounting for water treatment avoided through water reuse strategies available with decentralized systems.


This article deploys the Wastewater-Energy Sustainability Tool (WWEST), discussed in Stokes and Horvath (2010), to conduct a system-wide life-cycle assessment (LCA) comparison of centralized and decentralized wastewater systems.