Perhaps the majority of meaningful social interactions involve opportunities for cooperation juxtaposed with dangers of conflict. Brutus and Caesar, Jung and Freud, Lennon and McCartney all had famously productive friendships preceding their infamously nasty splits. Even the closest of relationships-those involving family members-are occasionally staging grounds for marital spats, sibling rivalries, and parent-offspring conflicts (Trivers, 1974). There are numerous historical instances of rulers, such as Edward IV of England and Selim I, Sultan of the Ottoman Empire, who ordered the executions of their own brothers. And even informal interactions between casual friends, such as an invitation to dinner or a party, involve various trade-offs, and may require deciding between a mini-betrayal and a minor self-sacrifice. Do you stick with your commitment to go to a distant coworker’s birthday dinner or skip it when a better friend invites you out? More generally, how do people come to decisions about prioritizing their own immediate interests or those of their acquaintances, friends, or families?