How selfish does our evolutionary history suggest that humans will be? We explore models in which groups are formed and dissolved and where reproduction of individuals is determined by their payoffs in a game played within groups. If groups are formed ``randomly'' and reproductive success of group founders is determined by a multi-person prisoners' dilemma game, then selfish behavior will prevail over maximization of group payoffs. However, interesting models exist in which ``group selection'' sustains cooperative behavior. Forces that support cooperative behavior include assortative matching in groups, group longevity, and punishment-based group norms.