To explain why cooperation with nonrelatives arises and persists in populations, British researchers developed a computational model in which players have varying degrees of cooperativeness (a willingness to allow partners to accrue benefits at their own expense) and choosiness (a willingness to leave partners based on their teamwork). After each round of play, an individual receives a payoff that reflects the effort both teammates exert. Individuals do best, however, if they manage to get their partners to do most of the work. After the payoff, players can continue together to the next round or decide to divorce the other, in which case each would be randomly paired with another partner. Because players that are repeatedly divorced get slapped with greater costs than those that stick together, cooperation and choosiness rise in tandem when many rounds are played—the equivalent of long life spans. Choose the January 10 Nature for the complete payoff.