In past decades, we have witnessed the widespread transformation of regional organizations (ROs). This transformation is marked by an increased institutionalization, such that ROs are no longer dominated by intergovernmental bargaining and instead enjoy greater independence. Institutionalization is reflected in the emergence of stronger dispute settlement bodies, independent bureaucracies, and international parliaments. In parallel, ROs are increasingly characterized by pooled decision making and access for transnational actors.

Interorganizational Diffusion in International Relations: Regional Institutions and the Role of the European Union, by Tobias Lenz, is motivated by this observation and asks the important question of what role the European Union (EU) has played in this transformation. Lenz argues that the EU “has systematically shaped the process of creating and changing regional institutions in other parts of the world” (p. 9). The book builds on a diffusion framework, viewing ROs as a product of interdependent decision making. Lenz’s argument comprises three central claims. First, the EU’s influence entails active efforts to affect how regional actors negotiate institutional choices for their ROs. This active influence is rooted in EU policies that incentivize other ROs to adopt certain institutional forms. These policies shape how the EU negotiates interregional trade deals and cooperation agreements with other ROs, provides them with financial and technical assistance, and engages in ongoing political dialogues with regional policy makers. Second, the EU’s influence operates through passive channels. The EU indirectly affects the preferences and strategies of national governments who negotiate regional institutional arrangements by generating information for governments about the effectiveness of institutional choices (also known as learning) and the legitimacy benefits of certain institutional designs (emulation). Third, the EU’s influence is conditioned by the nature of the regional contract. Open-ended contracts are more susceptible to EU influence, while fixed contracts are less so.

The book tests these theoretical claims using a sophisticated and compelling mixed-methods research design. The large-N analysis (chap. 4) uses a sample of 36 ROs to show that the “EU’s engagement with other IOs (active influence) as well as the EU’s own institutional development (passive influence) is associated with higher levels of institutionalization in other ROs” (p. 110). Additionally, ROs based on “open-ended commitments are more susceptible to outside influence and display greater levels of institutional diffusion from the EU than ROs that are based on fixed contracts” (p. 110). Case studies further document the EU’s influence. Chapter 5 illustrates the EU’s active influence on the design of the Southern African Development Community’s dispute settlement body, while chapter 6 argues that the design of the Mercosur’s Permanent Review Tribunal was informed by the EU’s passive influence. Chapter 7 illustrates how the EU’s influence on ASEAN’s parliamentary institution was enabled by ASEAN’s open-ended contract while the absence of such influence over NAFTA was hindered by its fixed contract.

The empirical analysis follows two chapters dedicated to theory. While chapter 3 develops the theoretical argument, chapter 2 situates this argument within the broader debates on regionalism. Chapter 2 is a valuable comprehensive review that provides a synthesis of the intellectual history on ROs and structures the literature into three main strands: (1) accounts based on intraregional dynamics, (2) accounts based on extraregional or global dynamics, and (3) accounts based on interorganizational dynamics.

Lenz’s argument is situated within this third category and significantly advances this theoretical approach. Importantly, the book provides the first large-N empirical test of interorganizational diffusion among ROs. Also, the book provides new theoretical insights by specifying the active and passive pathways of diffusion. It connects these pathways of diffusion to the negotiation of ROs, suggesting that the EU changes how regional actors bargain over their own regional institutions. Ultimately, this book, more clearly than previous works, brings politics into a diffusion framework and shows how bargaining over institutional choices occurs in the shadows of the EU.

There are some gaps in the analysis. One question that the book raises is why the EU has invested in actively influencing the design of ROs. Lenz claims that the EU does so to legitimate its own institutional innovations (p. 62). The book however does not test this claim or consider other possible motivations. For example, the EU may promote certain institutional forms elsewhere to generate material benefits for European interests in the EU’s dealings across the globe. Independent judicial bodies can secure property rights for EU investors across the globe while independent bureaucracies and access for transnational actors may provide European interests with greater opportunity to mold ROs’ policies to their benefit. Further inquiry is needed to determine the EU’s motivations for shaping regional institutions elsewhere.

Another gap in the analysis revolves around why the EU is the institutional pioneer to consider. Lenz assumes that it is because the EU is the most effective and legitimate RO and it has adopted policies of active influence. Left unexplored is whether the EU is unique in its active influence and why other highly effective and legitimate international organizations (IOs) have been less influential. Without addressing this issue, the argument becomes susceptible to charges of Eurocentrism and neglects the structural benefits the EU enjoys toward having such influence. For example, the EU has benefited significantly from the colonial legacies of its member states across the globe. This is important because many regional projects are intimately related to their colonial past. To take an example with which I am familiar, the East African Community (EAC) as it exists today is connected to colonial rule in Eastern Africa (see Richard Mshomba, Economic Integration in Africa: The East African Community in Comparative Perspective, 2017), while its court bears some connections to its colonial predecessor (see Bonny Ibhawoh, Imperial Justice: Africans in Empire’s Court, 2013). Put differently, has the EU been a model, not only because of its effectiveness, legitimacy and policies, but also because it enjoys structural power arising from the colonial legacies of its member states?

Relatedly, the book seems to sidestep whether other institutional pioneers influenced the EU’s “innovations.” For example, the European Court of Human Rights has a longer history of direct access for private litigants than the European Court of Justice, and the Parliamentary Assembly of the Council of Europe (previously the Consultative Assembly) preceded the precursor to the European Parliament. The EU is also not the first region to feature a common currency and central bank. For example, regional integration in Eastern Africa under colonial rule had a common currency regulated by a currency board (see Mshomba 2017). These examples beget the question of whether the EU’s “innovations” are themselves products of interorganizational dynamics.

Finally, the EU’s influence on interorganizational diffusion appears to be mostly a post-Cold War development. Yet it is unclear how the Cold War, and the related differences in power structures and the institutional environment, limited interorganizational diffusion. Was the EU’s institutionalization itself impeded by the Cold War, thus dampening its influence, or were opportunities for regionalism elsewhere hindered by the Cold War’s dynamics? Either way, these possibilities may hint at potential preconditions to the EU’s influence.

These gaps should be read as opportunities for future inquiry that are made relevant by this notable contribution. This book is a must-read for those who study regional organizations, the EU’s global impact, and the design of IOs.