Humans can engage in relatively indiscriminate altruistic behaviors such as donating money to charities, giving blood, and volunteering to review scientific papers. They also live in societies characterized by examples of cooperation of unmatched complexity, such as organized armies, the cooperative building of infrastructures such as roads and railways, and tax-paying, among others. (We exclude, of course, the “anonymous” societies of some social insects in which the animals themselves are not aware of the cooperative roles they play.) The emphasis on these unique aspects of our behavior1 has sometimes distracted scientists from paying attention to the more common aspects of our daily lives, which share characteristics with those of our fellow primates. We invite friends for dinner, console others after a loss, intervene in ongoing fights, and even groom others.2–4 These small acts of altruism, which constitute a large part of our daily social life, tend to resemble those of nonhuman primates.