Aggressive contests between social groups are widespread among primates and though they vary dramatically in their patterning, the sources of this variation are poorly understood. The identity of participants, their motivations, and the resulting group-level behaviors vary over time, across populations and among species. I introduce a framework for examining these aspects of intergroup conflict and present novel hypotheses to explain patterns of food and mate defense, focusing on the tension between male and female priorities. Moreover, it is widely assumed that such conflicts constitute a collective action problem: i.e., that only participants incur the costs of fighting but the benefits of successful defense are shared with defectors. I test this assumption in a wild population of redtail monkeys, using non-invasive measures of energy balance and stress to measure the costs and benefits of participating and defecting. Current evidence indicates that while benefits are shared, defectors face far greater costs than participators that preclude their involvement.