Despite what you might think from the piles of dishes accumulating in the laboratory sink, humans seem to have an innate tendency to cooperate with one another, even when it goes against their rational self-interest. Some theorize that the ability to cooperate is one of the main reasons humans have managed to survive in almost every ecosystem on Earth, but it poses puzzles for evolutionary biologists: Are cooperative urges ingrained in our genes? Or are we taught by our culture to play well with others? Are those who break the rules deviant, or are those who follow them driven by outdated urges that don't make sense today?
Recent studies of how humans and other primates cooperate have stoked these old debates with new data. Evolutionary theorists are attempting to piece together the forces that might have shaped cooperation. Neuroscientists are getting into the mix as well, identifying circuits in the brain that respond to cooperators and cheaters.
At first glance, cooperation seems to be an evolutionary anomaly. In the hardscrabble competition for food, territory, and mates, why would one individual go out of its way to help another? Nevertheless, the animal world has plentiful examples of cooperation that seem to be hard-wired: bees that collect pollen for the whole hive, mole rats that build elaborate tunnels used by other group members, and meerkats that risk their lives to guard a common nest.
Biologists have successfully explained why such behaviors can be beneficial in the game of passing genes to the next generation. In the 1960s, the late evolutionary biologist William Hamilton developed a theory of kin selection that showed how helping relatives can increase the chances that one's own genes will be passed on through them. The theory elegantly explains the behavior of cooperative insects, for example.
In the 1970s, evolutionary biologist Robert Trivers, now at Rutgers University in New Brunswick, New Jersey, developed a theory that explained why unrelated individuals might also be inclined to help one another. According to his theory of reciprocal altruism, helping a nonrelative increases one's own fitness as long as the recipient can be reasonably expected to return the favor. An innate willingness to help those who would help you back, he argued, would have been beneficial in early human societies, when people lived in relatively small groups and could keep track of who cooperated with whom.
Evidence for the benefits of reciprocity is starting to show up in studies of nonhuman primates. For example, primatologist Joan Silk of the University of California, Los Angeles (UCLA), and her colleagues reported last fall (Science, 14 November 2003, p. 1231) that social female baboons have a greater chance than less social females of having an infant survive to its first birthday. Somehow, grooming and staying in close contact with other group members seems to give female baboons a reproductive advantage. It's possible that the social butterflies are simply healthier overall and therefore can afford to be more generous, but Silk and her colleagues argue that the...