Ninety-five freshmen each recruited three peers to play a "group bidding game," an N-person prisoner's dilemma in which anyone could win movie tickets depending on their scores in the game. Prior to playing, all participants completed a measure of prosocial value orientation. Replicating and extending earlier findings (Sheldon and McGregor 2000), our results show that prosocial participants were at a disadvantage within groups. Despite this vulnerability, prosocial participants did no worse overall than asocial participants because a counteracting group-level advantage arose for prosocials, who tended to be concentrated in groups. Implications of this assortative process for the egoism/altruism debate, and for hierarchical selection theory, are discussed.