The rapid expansion of international non-governmental organizations (INGOs) numbers in the last half-century is usually explained to be a result of decolonization, globalization, and/or increase in the number of global issues. One additional hypothesis, which has not been discussed in the political science literature, is suggested by the demographic-structural theory. According to this hypothesis, the acceleration in INGO numbers was caused by the post-war baby boom and a crisis in the credential system. This study finds that cyclical increases in INGO numbers were preceded by expansions in the 30–39 cohort. Interestingly, the mean age of leaders across 12 international governmental institutions also oscillated, but with a lag (thus correlated with expansions of the 55–64 cohort). Thus evidence supports the idea that demographic-structural mechanisms contributed to the surge in INGO numbers during the last 50 years as a by-product of intraelite competition.