One well-studied factor that biases toward cooperation is genetic relatedness

One well-studied factor that biases toward cooperation is genetic relatedness. Familial ties are the driving force behind a large proportion of cooperative behaviors in animals. For example, individuals of some social insect species display such an outlandishly high degree of cooperation and altruism that most of them forgo the chance to reproduce and instead aid another individual (the queen) to do so. The late W.D. Hamilton, one of the giants of science, revolutionized thinking in evolutionary biology by explaining such cooperation in terms of the astoundingly high degree of relatedness among an insect colony's members. And a similar logic runs through the multitudinous, if less extreme, examples of cooperation among relatives in plenty of other social species, such as packs of wild dogs that are all sisters and cousins and that regurgitate food for one another's pups.

Another way to jump-start cooperation is to make the players feel related. This fostering of pseudokinship is a human specialty. All sorts of psychological studies have shown that when you arbitrarily divide a bunch of people into competing groups (the way kids in summer camp are stuck into, say, the red team and the blue team), even when you make sure they understand that their grouping is arbitrary, they'll soon begin to perceive shared and commendable traits among themselves and a distinct lack of them on the other side. The military exploits this tendency to the extreme, keeping recruits in cohesive units from basic training to frontline battle and making them feel so much like siblings that they're more likely to perform the ultimate cooperative act. And the flip side, pseudospeciation, is exploited in those circumstances as well: making the members of the other side seem so different, so unrelated, so un-human, that killing them barely counts.

One more way of facilitating cooperation in game-theory experiments is to have participants play repeated rounds with the same individuals. By introducing this prospect of a future, you introduce the potential for payback, for someone to be retaliated against by the person she cheated in a previous round. This is what deters cheaters. It's why reciprocity rarely occurs in species without cohesive social groups: no brine shrimp will lend another shrimp five dollars if, by next Tuesday, when the loan is to be repaid, the debtor will be long gone. And this is why reciprocity also demands a lot of social intelligence--if you can't tell one brine shrimp from another, it doesn't do you any good if the debtor will actually still be around next Tuesday. Zoologist Robin Dunbar, based at University College London, has shown that among the social primates, the bigger the social group (that is, the more individuals you have to keep track of), the larger the relative size of the brain. Of related interest is the finding that vampire bats, which wind up feeding one another's babies in a complex system involving vigilance against cheaters, have among the largest brains of any bat species.