Introduction

The study of informal international institutions has advanced significantly in recent years. New studies have explored numerous varieties of governance that occur outside of the traditional boundaries of international law and often take unconventional forms. They have sought to understand why states create and use these sorts of rules and bodies, how they evolve, and how they reshape world politics. Our research has focused on an important feature of this changing international landscape—informal intergovernmental organizations (IIGOs)—and we are pleased to see how others have advanced this line of research. The recent special issue of International Politics on the “cascading dynamics” of informality represents an important effort to deepen our understanding of these organizations, exploring a variety of new processes and institutions (Cooper et al. 2022). It also, in part, casts itself as a reaction to our common approach to studying informal institutions and the claims we have made.Footnote 1 The article by Cooper (2022) and the introduction by Cooper et al. (2022) offer a wide-ranging critical discussion of our work. We see such criticism as an essential way in which social scientific debates progress and therefore appreciate and value this constructive engagement. In that spirit, we wish to respond to several points, clarify some of the key research issues at stake, and outline ways of moving forward.

Ultimately, we believe that our work is highly complementary to that of Cooper, his co-editors (Parlar Dal and Cannon), and the authors in the special issue. We strongly agree with the core motivations behind the special issue and its central claim regarding the increasing importance of IIGOs (Vabulas 2019). That has been an important element of our research for over a decade and is systematically documented in our data (Vabulas and Snidal 2013, 2021; Roger 2020; Roger and Rowan 2022a, b). We have also learned a great deal from the research published by Cooper and the others included in the special issue. Collectively, they have enriched our understanding of many informal institutions, from the Group of Twenty (G20) and BRICS to the Quad and the Association of Southeast Asian Nations (ASEAN) (Cooper and Pouliot 2015; Cooper and Farooq 2013; Parlar Dal and Dipama 2022; Caballero-Anthony 2022; Cannon and Rossiter 2022; Kirton and Larionova 2022). Yet we do not agree with all their conclusions. There are important differences in emphasis, approach, and interpretation, several of which we highlight in what follows. Unfortunately, Cooper et al.’s criticisms often characterize our research in a way that unnecessarily heightens these differences. Our goal here is to outline the claims with which we disagree, while also stressing the mutual complementarities across different research traditions.

One of Cooper’s main arguments is that our “rationalist” approach to informal institutions employs concepts that limit our understanding of informal cooperation. Specifically, he contends that rational institutionalist (RI) approaches to informality and efforts to quantify IIGOs lead to a homogenization of phenomena across cases and circumstances that lacks crucial “nuance” (Cooper 2022: 13,16) and ignores “historical context” (Cooper 2022: 1, 3). To some extent, we agree: The purpose of theorization and conceptualization is to uncover generalizable similarities across phenomena whose specific features make them seem unconnected. An important aim of our research has been to show systematically that IIGOs are a different and increasingly important form of institutionalization, distinct from others that have already been studied quite thoroughly—formal intergovernmental organizations (FIGOs), in particular. IIGOs should be studied together because they share common properties that enable a clearer understanding of what is and is not the “same” across seemingly different cases. But while there are important similarities across IIGOs, they also vary and change—just like all organizations. Uniqueness, however, can only be properly understood in a comparative manner. Clarity in concept and theory is essential to understanding differences within and across bodies, whether that be in terms of structural variation or the causal dynamics that drive it.

Cooper and others involved in the special issue also suggest that RI researchers have historically regarded formal institutions as analytically more important, that we are only just “catching up” with recent trends, and that our work is too “US-centric.” This is largely wrong. RI has never regarded formal institutions as obviously “superior.” Further, as we show below, RI began with purely decentralized cooperation and has a long tradition of studying informal institutions. Rather than underplaying or ignoring informality, RI offers a rich set of tools for understanding it. The claim about US-centrism, in turn, broadly relates to questions about the agency and role of the US in our accounts, suggesting that we focus excessively on cases from the Global North and therefore make causal and descriptive claims that do not reflect the dynamics at work across institutions. Here, it is true that some of our initial interest in IIGOs was (like Cooper's) sparked by the importance of G-groups such as the Group of Seven (G7) and the G20 in which the US and Global North have played a prominent part. Yet, overall, our goal has been to build theory and assemble data sets covering all IIGOs across all regions, including the Global South. Where our theory or data have been constrained, we have been explicit about such scope conditions and empirical limits.

Looked at in this way, the research contained in the special issue is not at odds with our approach. It helps to fill gaps in our understanding, it answers unresolved questions, and it collectively broadens the research agenda. Many of the criticisms made of our work are not inherent to analyses that adopt an RI approach. Rather, they are a product of the specific questions that have been asked and the early stage of research on informal institutions. Progress will undoubtedly be made as more scholars build on these foundations. RI can be usefully informed by the qualitative findings reached by Cooper, Parlar Dal, and the other authors included in the special issue. But, equally, we believe that research on informal institutions cannot reach its full potential without connecting to some strong theoretical tradition—of which RI is a prime candidate and largely compatible with other leading alternatives.Footnote 2

In this article, we begin by addressing Cooper’s general critique of RI approaches and how his narrow interpretation of RI work sidesteps or underplays important findings related to IIGOs. We highlight our own work, underscoring both empirical and theoretical contributions to date and foreshadowing ongoing research in this regard. Next, we consider several of Cooper’s more specific arguments related to conceptual issues and empirical variation across institutions. On this front, we discuss our efforts to measure IIGOs and recent work that unpacks important organizational variation across them, and shows that different informal institutions operate and evolve quite differently. Following this, we address criticisms of US-centrism and RI’s alleged failure to engage the Global South by discussing issues of representation in the field, the historical cases and contemporary vignettes we have examined, and the empirical claims we have made. In the conclusion, we propose ideas for future research on IIGOs and emphasize the importance of moving beyond paradigmatic debates by taking full advantage of the synergies among existing approaches.

Rational institutionalism and study of IIGOs

Cooper uses RI as a label for our research—and it is a major unifying feature of our work—but it is not always clear what he means by the term. Above all, we conceive of RI as a way of thinking about how actors make decisions about institutions and engage in strategic action within institutionalized environments (Abbott and Snidal 2021; Rowan and Snidal 2023). RI begins from the idea that agents engage in consistent goal-seeking to explain the choices of different actors, their interactions, the influence of the broader contexts in which they operate, and how agents can change their environment by creating and working through institutions of various types. The RI approach has been used to study a diverse range of phenomena in International Relations (IR), and has been especially fruitful for analyzing international cooperation (Keohane and Martin 2003). Indeed, RI has highlighted the importance of many factors, including problem structures, information, technology, domestic politics, the distribution of power, and institutional design features (Koremenos et al. 2001). While its basic assumptions about decision-making are parsimonious, RI also leaves room for an array of motivations, from purely selfish economic or security-related objectives to more altruistic, other-regarding ones. Further, although RI operates from a distinct starting point, it does not reject the possibility of non-instrumental behavior. Instead, RI and constructivist approaches are best seen as complementary rather than competing perspectives (Fearon and Wendt 2002).

Conceived in this way, RI offers a systematic yet flexible approach to the study of IR and has much to say about informal institutions. From its inception, RI scholars have conceptualized institutions as including both formal and informal varieties. Snidal's (1985: 923) first major piece on the topic took off from the observation that institutions can range from “formal organizations […] through a wide variety of intermediate institutions to very informal methods of collective organization.” The contributors to Krasner's (1982) pioneering volume on international regimes analyzed both formal and informal arrangements, and a number focused on what Vabulas and Snidal (2013) would later term “IIGOs.”Footnote 3 Jervis's (1982) classic piece on security regimes, for example, analyzed the Concert of Europe—arguably, the original “G-group.” Around the same time, Putnam and Bayne (1984) highlighted the importance of more recent cases of summitry—the G7, in particular—while other work, including Oye's (1986) volume on “cooperation under anarchy” and Lipson's (1991) research on informal agreements, had explicit emphases on decentralized cooperation based on extremely low levels of institutionalization and non-binding cooperative instruments.

This body of work constitutes what Tieku (2021) refers to as a “first generation” of informal IR studies. Subsequent RI research on the dynamics of “legalization” and “soft law” pushed this research forward considerably by more explicitly exploring choices about the form of cooperation (Abbott and Snidal 2000; Shaffer and Pollack 2010). In doing so, a variety of causal factors have been identified. For example, Vabulas and Snidal (2013) argue that a need for flexibility, speed, and confidentiality linked to features of an underlying problem structure—that is, the strategic nature of the issues that states hope to solve—can often explain choices about the informality of IGOs. Roger (2020), in turn, calls attention to domestic politics and institutions, especially within powerful states able to dictate the terms of cooperation. In his account, growing political constraints, and the involvement of independent agencies in state decision-making, are crucial drivers behind the rise of IIGOs. More recent studies by Vabulas and Snidal (2020), Manulak and Snidal (2021), and Roger (2022a, b) have developed related arguments about changing structures of power, the role of technology, and the relationships between formal and informal IGOs as important factors for understanding choices about informality.

The diversity of mechanisms identified by RI scholarship and its long engagement with questions of informality speak to several of Cooper’s criticisms. First, his claim that RI has “underplayed” informal institutions, has emphasized “the primacy of formal multilateral organizations,” and “has not given pride of place to this trend until recently” are mistaken (Cooper 2022: 10). As already seen, RI scholars were among the first to explicitly theorize informal institutions and have regarded both formal and informal varieties as important features of world politics. Neither is “privileged” (Cooper 2022: 11, 19). Both formal and informal institutions also feature prominently as cases in the RI literature. The Concert of Europe, the G7, AOSIS, BRICS, the Helsinki Accords, and the GATT are analyzed alongside the UN, IMF, and NATO. In fact, Abbott and Snidal's (1998) work on the dynamics of FIGOs—which Cooper points to in support of his claim—was a reaction to an earlier over-emphasis on regimes and informal, decentralized cooperation. Yet, this attention to formal IGOs was not a rejection of informal institutions and there was no claim that formality was generally “superior” (Cooper 2022: 4). Indeed, Abbott and Snidal's (2000: 423) contemporaneous work on soft law had a central aim of showing “that international actors often deliberately choose softer forms of legalization as superior institutional arrangements.”

Second, while RI research begins from a common set of assumptions, it does not reduce empirical patterns of informality to a “uniform account,” as Cooper et al. (2022: 6) suggest. Where conditions vary, RI expects to see different processes play out. Accordingly, decision-making about informality may be quite distinct across time periods, issue areas, and geographic regions. Our work on IIGOs, as pointed out above, identifies many causal mechanisms—and not simply functionalist ones—that vary in importance across cases. Equally, we specify scope conditions that determine when they operate. The specific drivers set out by Vabulas and Snidal (2013) vary considerably in their relevance across cases, as states in different regions and issue areas confront different problems. Vabulas and Snidal (2020) address the dynamic way in which IIGOs help states manage power transitions by providing flexible institutional arrangements that facilitate bargaining without freezing outcomes in permanent institutions while the power distribution evolves. Roger’s (2020) argument about the role of domestic politics and institutions is also not a “one-size-fits-all explanation” of informality that is applicable to all times and places (Cooper et al. 2022: 6). Rather, its dynamics are limited to advanced democracies and the postwar period. Beyond this—and among developing non-democracies, in particular—other dynamics may well prevail.

Third, as this indicates, RI pays careful attention to the historical and institutional contexts of cooperation. Empirically, for instance, our research has documented historical developments by analyzing the circumstances in which IIGOs have been created, drawing on a range of empirical sources—from archival materials to interviews—to generate a rich picture of the causal processes at work (Roger 2020). In addition, we collected systematic data on informal institutions to illuminate the changing terrain of global governance, both across states and over time (Vabulas and Snidal 2013, 2021; Roger and Rowan 2022b). An important finding is that informal IGOs have been with us for some time. And, even a quick look at our data challenges the heavy emphasis that Cooper (2022: 2) and others place on the G7 as the “original source of a model that extended appreciably in the decades to come” and the global financial crisis (GFC) as the “major transition in this pattern of institutionalism.” The first IIGO or G-group (the Concert of Europe) emerged contemporaneously with the first FIGO (the Central Commission for the Navigation of the Rhine), for example, and numerous IIGOs were created prior to the 1970s. The rapid proliferation of IIGOs also started well before the GFC—largely coinciding with the end of the Cold War—and has carried on after that point, though with some variation across regions and issue areas (Vabulas and Snidal 2022).

Theoretically, RI research has also explicitly aimed to account for how different historical and institutional contexts matter for the uneven topography of informal cooperation. Beyond specifying scope conditions for different mechanisms, as described above, Manulak and Snidal (2021) show how historic shifts in communication and transportation technologies have shaped the supply of IIGOs by lowering the costs of interacting without FIGOs. Roger (2020) offers a dynamic account of the rise of informality in the post-war period, highlighting the importance of institutional changes within powerful states—especially growing polarization and the rise of the regulatory state—which have projected outwards to reshape the legal foundations of global governance. Additionally, Vabulas and Snidal (2013, 2023) have shown that while states have always jealously guarded their sovereignty, the recent availability of IIGOs has allowed them to pool decision-making without delegating authority to FIGOs.

Finally, Cooper suggests that RI presumes pure competition between types of institutions, but this is neither true of IIGOs nor of soft law more generally (see, for example, Shaffer and Pollack 2010). In fact, their relationships may be complex. When Vabulas and Snidal (2013) first introduced the IIGO concept, they theorized when and why states “use IIGOs to complement, counter or substitute for FIGOs.” They argued that to understand how informal IGOs operate we must examine their connections to formal ones. Later research by Roger (2022a, b) has also demonstrated that formal IGOs have frequently served as “coral reefs,” providing crucial support to informal institutions that can help to explain their prevalence across regions, time periods, and issue areas. In short, RI in no way implies that the relationship between formal and informal varieties of governance is zero-sum and these inter-institutional relationships have been fertile terrain for RI analyses.

Conceptualization and measurement of IIGOs

Beyond the theoretical underpinnings of our work, a second unifying feature has been careful attention to concepts and to measuring the world of IIGOs as rigorously as possible. When we started writing on the topic, scholars had acknowledged that many IGOs and other types of public institutions fell outside the formal model. References to “non-formal” organizations, for instance, can be traced back to some of the earliest writing on IGOs (Roger and Rowan 2022a). Work by Potter (1935) and Cox and Jacobson (1973)—and more recent studies by Volgy et al. (2009) and Klabbers (2001)—recognized that not all IGOs were based on treaties, and that many had “light” institutional footprints. However, as particular bodies like the G20, BRICS, and BCBS, drew increasing attention from scholars, it became clear that the conceptual tools and datasets available for systematically analyzing these informal arrangements were inadequate. Thus, to make sense of the changes that were afoot, and the ways in which cooperation varied, the introduction of the concept of an IIGO was essential.

Given that RI forces scholars to be precise about the properties of institutions, this necessitated tough decisions about how to conceptualize the institutional landscape. Which bodies should be grouped together, and which should be set aside because they vary in other significant ways? What, from the perspective of states and policymakers especially, was most important about these institutions and how did they differ from other, better understood bodies, such as FIGOs? The approach we have taken—which varies somewhat between us, as detailed below—has emphasized analytic clarity and generality. Cooper critiques this as conceptually “thin,” but thinness serves a valuable purpose here. Vabulas and Snidal (2013, 2021) established an ideal type definition of an IIGO, grouping together a wide set of organizations that are unified by a few key features that sharply distinguish them from an opposing ideal type of a formal IGO. The advantage of this approach is that it isolates the key properties of interest and, by tightly conceptualizing and sharply differentiating between organizational types, enables a better understanding of their central logics.Footnote 4 While ideal types are not real, this analytic precision helped to capture what was most relevant about informal bodies as such.Footnote 5 Without such clarity, it would be challenging to categorize and measure them systematically.

Bringing informal institutions together in this way—that is, by creating the concept of an IIGO and studying them as a distinct type of institution—was essential to focusing clearly on first-order questions: why do states choose informal designs and why have IIGOs become more common over time? In asking these questions, our aim was to emphasize the larger story behind the drivers of informality. From the outset, however, we have been aware of the need to account for differences among institutions. The most striking aspect of Vabulas and Snidal’s IIGO definition is that there is literally no secretariat, not even a secretary who might organize meeting rooms or just get coffee. This is a highly restrictive definition. Despite this stringent—and spare—requirement, they identified a substantial number of bodies, including many important ones, from the Concert of Europe to the G20. Inevitably, though, there is much variation across informal institutions, as Vabulas (2019) has described, in terms of their mandates, membership, administrative structures, and so on. Further, numerous bodies do not meet the strict standard they set—there are many “close calls” that possess some of the properties of IIGOs but only to a degree. Correspondingly, Vabulas and Snidal (2013) noted in their first piece that there is no bright line between formal IGOs and informal ones. Rather, international institutions vary along a spectrum of formality.

Roger and Rowan’s (2022b) definition of an IIGO is slightly broader than Vabulas and Snidal’s and demonstrates that RI scholarship has not been dogmatic when it comes to concepts. Their approach places more emphasis on the legal nature of an institution and includes bodies created by state organs operating below the executive level. They also allow informal IGOs with secretariats. This means that there are differences in the way that Roger and Rowan measure institutions and the degrees of variation they allow, which has implications for key empirical results (Roger and Rowan 2022a). But this alternative approach is not at odds with Vabulas and Snidal’s original formulation. It is, instead, driven by the need to tailor concepts to alternative research questions. Accordingly, our joint approach to conceptualizing and measuring IIGOs is but a first step in a larger research agenda. Our initial aim was to crystallize the issues at stake in debates about IIGOs, visualize the larger landscape of informal institutions, and to initiate a larger conversation about them. In this sense, the studies in the special issue of International Politics, and those appearing in other venues—plus, our own ongoing work—constitute a next generation of research on informal institutions.

Seen from this perspective, Cooper’s concern that we neglect “state-based plurilateralism”—which he appears to equate with global summits such as the BRICS, G7, and G20—is best understood not as a criticism of RI but as a call to push research on informality in a particular direction. He believes this constitutes the “core manifestation of informal institutionalism” and should “merit greater attention” (Cooper 2022: 5; Cooper et al. 2022: 9). But, set in the broader institutional context our research has illuminated, state-based plurilateralism is far from being the only variety and, should not be privileged. No doubt, it is an important institutional form that deserves careful analysis. Further, as this is done, our bet is that RI will offer a productive approach. In fact, many compelling and insightful studies of global summitry already are situated within this research tradition (Putnam and Bayne 1984; Putnam 1988; Odinius 2021). However, while we encourage further work on summitry along these lines—and from entirely different standpoints, e.g., practice theory and other sociological approachesFootnote 6—this should not lead us to neglect the rest of the informal governance landscape.

Methods, cases, and scholarship

Our final reflections relate to methodological issues and how scholars should go about studying informal institutions. In our research, we have generally adopted a mixed methods approach, placing considerable emphasis on the collection and analysis of quantitative data while also exploring theoretical conjectures with detailed case studies. As already stated, a quantitative approach is valuable because it forces us to be precise about what we observe: what differentiates formal and informal IGOs and what distinguishes one informal institution from another. It also allows us to see broader patterns across issue areas, regions, and time periods. Assembling systematic descriptive data has allowed us to develop a rigorous picture of the new terrain of global governance and identify interesting new questions to explore. Equally important, this effort leads to better answers. The collection of quantitative data unlocks our ability to undertake statistical analyses to evaluate the causal mechanisms that we and others have advanced. Our understanding of IIGOs has been considerably enhanced as a result.

At the same time, quantitative work by itself is insufficient, so we have typically combined it with qualitative analyses. Datasets provide insights into broad patterns of variation and statistical analyses are particularly useful for assessing the generalizability of hypotheses. Quantitative research can also guide and situate qualitative work. Ultimately, though, it cannot do justice to the unique elements of individual cases and, more importantly, is less useful for detailed examination of causal processes, as Cooper points out. To elucidate the mechanisms at play, we have also tested our ideas via an array of detailed qualitative analyses (or, when there are space constraints, shorter vignettes). These qualitative “deep dives” include Roger’s (2022a, b) studies of the GATT, the Basel Committee, IOSCO, and the International Competition Network. Vabulas and Snidal’s (2013, 2021) qualitative analyses have, in turn, examined inter alia the Australia Group, AOSIS, ASEAN, the Concert of Europe, the BRICs, the G7, and the G20.

Despite this diversity, Cooper et al. (2022) have criticized the approach taken by RI scholarship and some of our specific methodological choices. In particular, they regard the literature as being excessively “US-centric” (Cooper 2022: 1, 3, 9, 19)—and as lacking an “anticipatory component.” Here, they have a few things in mind. First, they believe our work has placed too much emphasis on the United States (US)—giving it “prime agency” (Cooper 2022: 3, 9) and neglecting the role of the Global South. This is, partly, a causal claim: That we overlook how the agency of others is also important for the move to informality. But it is also a methodological one: That the cases we have focused on are drawn from the Global North and can therefore tell us little about informal institutions created elsewhere.Footnote 7 Second, they charge that the RI literature—our work included—has generally been too focused on studies conducted by mainstream US scholars and neglects the “ground-breaking scholarship coming out of the Global South” (Cooper et al. 2022: 8). Finally, despite suggesting elsewhere that RI researchers tend to be too ambitious in terms of the claims they make, they argue that our work is too cautious and fails to offer predictions about future trends.

These arguments should be taken seriously. We concur, for instance, that debates about informal institutions have played out among too small a group and need to be “opened-up” to more voices. In recent years, scholars including Acharya and Buzan (2019) and Zvobgo and Loken (2020), have highlighted IR’s considerable bias toward “Western” and largely White voices, advancing powerful calls for embracing a more “global” IR. Recent research by Roger et al. (2022) confirms these patterns, highlighting the imbalances that exist in research on global governance. Thus, there is significant work to be done in IR in general and in the field of global governance more specifically. In this sense, we commend the special issue for encouraging a more diverse set of perspectives.

Related to this, we agree that more effort is needed to understand informal institutions beyond those in the Global North, and how these matter for the patterns we observe. In this regard, the special issue also makes a powerful contribution. Where we differ is that we see this need as a product of the way the research agenda has evolved, not a problem for RI per se. Further, when the objectives of our early research and the scope of our claims are properly considered, charges about US-centrism have less merit. At the outset of our research on IIGOs, some of the most relevant studies had largely focused on explaining regional contrasts between the Global North and South. Institutions in the Asia–Pacific region, specifically, had often been noted for their relative informality, and scholars like Acharya (2001), Kahler (2000), Jetschke (2009), and Ravenhill (2001) had approached this by explaining why they differed from the much more legalized structures that prevailed in Europe and the Transatlantic area more generally. They did so from different theoretical vantage points: strategic, largely RI-aligned accounts and more sociological ones based on different regional cultures—e.g., the “ASEAN way”—and mechanisms of diffusion or imitation. Our early research found, however, that the contrast at the heart of this literature was at least partly mistaken, and that the central research question about informality had to be reframed.

In fact, our IIGO data showed that while cooperation among Asia–Pacific states was relatively informal, it was in the Global North where the shift to informal institutions had gone the furthest (Roger 2020; Roger 2023; also see Vabulas and Snidal 2022). Roger (2020) focused on these dynamics. Given that Transatlantic states had been the chief advocates of an earlier “move to law” (Goldstein et al. 2000: 1), why were they now relying relatively more on IIGOs as softer instruments of international cooperation? In Roger’s account of this historic shift, the agency of the US and other Western states rightly deserve emphasis. It is important to remember, however, that Roger’s explanation was not intended to offer a “catch-all” account, explaining informality everywhere and whenever it has occurred. It was carefully hedged by scope conditions and the degree to which this explanation could extend to the Global South was left uncertain, acknowledging that further research was needed.

That said, RI analyses have hardly set the Global South aside. While Vabulas and Snidal’s (2020, 2021) work was also partially motivated by the growing importance of IIGOs like the G7 and G20—where the US and European states have dominated—they have consistently offered arguments and analyses that are more general in nature. In contrast with the claims by Cooper et al., they have explicitly attempted to understand the processes operating in non-western cases including the BRICS, AOSIS, and ASEAN. Although the studies in the special issue are therefore not unique in terms of their focus on such bodies, they help to complete the picture RI scholars have painted by grappling with additional issues these institutions raise. As these debates advance further, we expect that RI will continue to have an important place. While differences of history, culture, and development may lead to varying institutional choices, our central point is that there are also common circumstances and conditions that explain why informal institutions operate within the Global North and Global South, as well as across them.Footnote 8 Even when states in the Global South embrace informal institutions for different reasons, the value of RI is still significant. The design and dynamics of institutions may be driven by different situation structures, levels of trust and information, domestic political structures, and institutional environments. But, despite all these differences, RI forces us to take the agency of actors seriously and challenges any supposition that “Southern” decision-making is any less carefully calculated.

Thus, RI offers a powerful set of tools—though certainly not the only ones—for understanding past and contemporary patterns of cooperation. But what about the future? Cooper et al. (2022: 9; Cooper 2022: 11) criticize RI for lacking a “predictive capacity” about future trends, claiming that there is “caution built into this type of scholarship.” On this front, while it is true that we have not attempted to predict future developments, as our understanding of informal institutions improves we do believe that RI provides firm foundations for studies, which, with care and modesty, may offer a basis for prognostication. Much more importantly, it provides evidence and ideas regarding the institutional possibilities that are available as states confront governance challenges.

Conclusion

Cooper points out lacunae and challenges that RI should address when studying informal institutions. We embrace many of his key points and have long been aware of the limits inherent in “the use of simple models to understand a complicated world”(Snidal 1985: 926). Nonetheless, we wish to underline the importance of RI for understanding the drivers, dynamics, and effects of informal institutions. As we have shown, RI research provides a strong basis for the analysis of IIGOs and other informal governance arrangements and has laid much of the groundwork for an ambitious research agenda. Its value is not impugned simply by pointing to issues it has not tried to account for (yet). No doubt there is much further work to be done, and RI and other theories will need to be adapted to understand new questions about how informal institutions work, evolve, and affect states. But, as scholars address those questions more fully, RI will continue to offer valuable tools for the next generation of researchers.

Analytically, our research combines insights from cases with broader generalizations. Conceptually and empirically, it has been premised on the idea that the diversity in IIGOs is best understood by first identifying commonalities across institutions—which unite them within in a single category—and then understanding the deviations that we see in specific cases, where a much more fine-grained variety of analysis is necessary. Future studies in the field will certainly show that a variety of additional factors may be at play, and these will undoubtedly vary with the contexts, actors, and problems confronted by states in different parts of the world. But this research should be richer for having the comparative, systematic foundations our work has developed.

We share Cooper, Parlar Dal, and Cannon’s overarching goal of better understanding informal international institutions. On that basis, we welcome the special issue. Yet, we regret the effort to heighten differences with our work, separating approaches that we think should be joined. The articles in the special issue, in fact, offer good examples of how we can deepen our understanding of the internal dynamics of individual cases, and they are better for being situated in a more general understanding of the spectrum of international institutions identified by RI. Rather than creating new divisions, thens, our common goal should be to combine the respective strengths of different research traditions and methodological approaches to advance our common understanding of informal institutions as one of the most important contemporary forms of global governance.

The many exciting avenues for future research will require widening and sharpening our analytical lenses. As the literature on informal institutions has grown, scholars have identified a seemingly ever-expanding number of variants and sub-variants—some of which, like ad hoc coalitions (Reykers et al. 2023) and contact groups (Prantl 2006; Sauer 2019), may be even less institutionalized than those we have analyzed. Many IIGOs also evolve over time. After they are created, some are transformed into FIGOs (Vabulas and Snidal 2021), some establish links with other institutions (Roger 2022a, b), some attain new capabilities and sources of authority (Rodriguez Toribio 2022), some are replaced or die off, and, of course, many stay as they are (Vabulas and Snidal 2021). Finally, we are only just beginning to explore the impacts, effectiveness, accountability, and legitimacy of these institutions. Our work has helped to map this institutional terrain and has described numerous causal dynamics and cases, answering several fundamental questions. But many puzzles remain. And, moving forward, the rich set of questions that scholars confront will be best answered by drawing on an array of methodological and theoretical approaches—Rational Institutionalism included.