Do international organizations (IOs) have an independent effect on state preferences? Despite the importance of this question in the international relations literature, isolating the causal effect of IO membership has proved difficult because of endogeneity concerns. This article offers a novel empirical approach to identifying the causal effect of shared membership in IOs on member state preferences. We exploit the fact that states joining the European Union obtain automatic membership in several other IOs through the European Union’s membership in these organizations. We then use a difference-in-differences strategy to estimate the effect of automatic membership in IOs on preference similarity with other members versus preference similarity with nonmembers. We demonstrate that shared IO membership leads to an increase in preference similarity. This approach offers a useful way to disentangle the effect of IO membership from selection effects that lead states to join IOs.
Egel, N., & Obermeier, N. (2023). A Friend like Me: The Effect of International Organization Membership on State Preferences. The Journal of Politics, 85(1), 340–344. https://doi.org/10.1086/722348
