This paper aims to explain a major barrier to societal integration: the remarkable homogeneity of voluntary associations. The explanation derives from an ecological theory of voluntary affiliation that asserts that organizations compete for members in a property space defined by the sociodemographic characteristics of members. Voluntary organizations lose fastest those members who are either atypical of the group (the niche edge hypothesis) or subject to competition from other groups (the niche overlap hypotheis). The authors analyze an event-history data set, generated by the life-history calendar approach, of 2,813 voluntary association membership pells. The results, which strongly support both the niche edge and niche overlap hypotheses, substantiate the competitive ecological model of group structure.