This article specifies a simple mechanism – a behavioral norm defined in ethnic terms – to understand the dynamics of mass participation by reluctant Hutu in the 1994 Rwandan genocide. The mechanism, which has not been analyzed systematically in the scholarly literature, is specified using an agent- based model of within-group interaction that captures the role of intra-Hutu coercion in precipitating genocidal violence, yet is general enough to be applied to other group conflicts and contexts. The model highlights the role of individual heterogeneity, within-group punishment, behavioral adaptation, group networks, and ethnic entrepreneurs, and generates a set of results on norm formation and change, a number of which are not intuitive.