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For a long time, international organizations (IOs) such as the United Nations, the European Union, and the OECD have played a prominent role in research on the diffusion of policies and institutional features among national governments. Some studies portray IOs as efficient fora, facilitating the exchange of knowledge between member state governments (Greenhill 2010; Needergard 2006; Holzinger, Knill, Sommerer 2008; Simmons and Elkins 2004). IOs can also take an active role by collecting and distributing information on good practice and by promoting particular regulatory instruments or norms (Finnemore 1993; Keck and Sikkink 1998; Risse, Ropp, and Sikkink 1999; Joachim 2003). And when a substantial degree of authority has been delegated to the international level, IOs affect decisions in member states by conditioning financial assistance or the admission of new members on the adoption of certain rules (Brooks 2005; Schimmelfennig and Sedelmeier 2004; Weyland 2007).

Diffusion, understood as a process where ‘prior adoption of a trait or practice in a population alters the probability of adoption for remaining non-adopters’ (Strang 1991, 325), is not the exclusive domain of national governments. For more than one century, research on diffusion has provided evidence for interdependent decision-making at different levels (Rogers 1995). In addition to nation states and their governments, diffusion scholars studied voters (Zaller 1987) and consumers (Rogers 1976), firms (Abrahamson 1991), cities (Shipan and Volden 2006), and local governments (Walker 2006) as well as states in a federal system (Gray 1973; Berry and Berry 1990). Diffusion is a relevant phenomenon for all kinds of institutions or organizations that face uncertainty, social pressure, and shared problems. Thus, it is reasonable to expect that interdependence matters for decision-makers in IOs as well.

Recent years witness a growing scholarly interest in this phenomenon. Grigorescu (2010) showed how formal connections between IOs lead to the spread of bureaucratic oversight mechanisms. Ovodenko and Keohane (2012) studied institutional diffusion in international environmental cooperation. Böhmelt and Spilker (2016) demonstrated how international interactions affect the design of environmental treaties. Sommerer and Tallberg (2019) provided an analysis of interdependence in the spread of non-state participatory arrangements across IOs. Alter (2012) studied the global diffusion of European style international courts, and Lenz and Burlikov (2017) showed how the EU as a model has shaped the evolution of regional institutions elsewhere.

Still, we know relatively little about diffusion processes among IOs. In this article, I will discuss a number of reasons for the relevance of this approach, basic features of an explanatory framework, and the consequences of diffusion among IOs. This article will end with an outlook on future trends and challenges in research on diffusion among IOs.

How Diffusion Became Relevant for IOs: Two Major Trends in Global Governance

Since the end of the Cold War, global governance has undergone fundamental changes. In the following, I will highlight two developments that contributed to the growing relevance of diffusion among IOs. First, international organizations are more important than they have been ever before. We have witnessed a growth in the number of institutions beyond the nation state, and, more importantly, a rise in the authority transferred to the international level. Member states delegate more and more tasks to supranational bodies, expand the use of majority voting, and continue to broaden the scope of IO mandates (Hooghe et al. 2017; Zürn 2018; Hooghe et al. 2019). One of the consequences of this trend are growing geographical and functional overlaps between IOs, increasing the likelihood of institutional cooperation and competition. In the field of security politics in Europe, for instance, we observe collaboration between NATO and OSCE (see also van Willigen and Koops 2015). High-level meetings like the EU-Arab League summit become more common. In 2016, the OECD initiated an IO partnership, including secretariats from more than 50 IOs ‘to foster dialogue on shared challenges and support common understanding on good practices in international rulemaking’. There are also signs of growing competition between IOs, as in the case of the Asian Infrastructure and Development Bank and other development IOs (Wang 2017; for the relationship between overlaps and interface conflicts, see Zürn et al. 2018).

Second, it has been argued that growing IO authority has led to a politicization of global governance (Zürn 2018). Today, IOs are under enormous pressure from different stakeholders. Once the realm of technocrats operating under the radar of public scrutiny, IOs like the World Bank, the IMF, and the WTO became the target of large-scale protests and campaigns during the 1990s. In recent years, challenges often come from member state governments questioning individual IOs – as in the prominent case of Brexit – or the existence of multilateral arrangements per se (Weiss 2018). Overall, the growing prominence of efficiency and performance reviews among states (Simmons and Kelley 2020) has also begun to expand to the world of IOs. Since 2010, the Aid Transparency Index ranks development aid agencies and IOs. And since 2002, the Multilateral Organization Performance Assessment Network (MOPAN), a network of like-minded donor countries, assesses the organizational effectiveness of IOs.

These challenges from key stakeholders questioning the effectiveness and legitimacy of IOs make diffusion more relevant and more likely. They urge IOs to legitimate their activities and structures. One way to deal with such pressure is to take a closer look at the past behavior of peer groups. Emulating what similar actors did in similar situations may give an organization more legitimacy, resources, stability, and enhanced survival prospects.

Mechanisms and Pathways of Diffusion Across IOs

The main goal of most diffusion studies is to find evidence for interdependent decision-making. In research on policy diffusion, two follow-up questions are particularly prominent: why policies diffuse and which pathways are relevant for the transfer of models and ideas from one actor to another (Graham et al. 2012). A large number of diffusion mechanisms have been discussed, which can be summarized in four broader categories: learning, emulation, competition, and coercion (Gilardi 2012; for alternatives, see Simmons and Elkins 2004; Graham et al. 2012). Learning refers to processes where decision-makers use experience from other actors or their own past. It can be based on the fully rational processing of information on successful models, or, given costs and limitations that human beings face, on cognitive shortcuts that facilitate decision-making. Emulation follows the logic of appropriateness: decision-makers........

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