What explains the large variation in the time taken by states to ratify the 1948 Genocide Convention? The costs of ratification would appear to be relatively low, yet many states have waited several decades before ratifying this symbolically important treaty. This study employs a “nested analysis” that combines a large-n event history analysis with a detailed study of an important outlying case in order to explain the main sources of this variation. Surprisingly, the results of our event history analysis suggest that states do not become more likely to ratify once the treaty has become widely adopted by others. We use the case of Japan to examine this relationship in more detail. We argue that once the norm embodied in a human rights treaty develops a “taken-for-granted” character, the rate of ratification can slow down because the marginal costs of additional ratifications begin to outweigh the expected benefits.