Human generosity is a bit of a puzzle for evolutionary psychologists. There are sound reasons for people to be generous with one another: They may be part of the same family, or work together in the same society. Over the years, though, experimental subjects have proven far more generous than those factors suggest they should be. In experiments with total strangers to whom they're unrelated, and whom they'll never see again, people are often surprisingly (and, from a theoretical point-of-view, needlessly) generous, cooperative, and unwilling to cheat. They are, in short, surprisingly nice.
Why should this be? There have been lots of explanations (naive, optimistic undergrads? a culture of friendliness and charity?), but none of them seem to provide the sort of long-term, structured pressures that might explain our friendliness evolutionarily. Now the evolutionary psychologists Andrew W. Delton, Max M. Krasnow, Leda Cosmides, and John Tooby say they've found the answer. Essentially, it's that every social encounter between two people involves a guess about whether or not you'll meet again in the future; you have to decide whether or not an interaction will be "one-shot" or "repeated." By modeling "one-shot discrimination" in a computer, the group has shown that it makes more sense to presume that you'll meet again down the road. Their paper, "￼Evolution of direct reciprocity under uncertainty can explain human generosity in one-shot encounters," has just been published in the Proceedings of the National Academy of Sciences.
All other things being equal, we seem to assume that we'll meet strangers again.
Their model is actually quite simple. A computer simulated a large number of 'agents,' which were capable of interacting with each other. During those interactions, some agents chose to cooperate, other to cheat. Cheaters were rewarded with more fitness points up-front, but, having cheated, were barred from interacting with the cheatee again; cooperators received fewer points, but kept options open about cooperating more in the future. Cheaters nipped relationships in the bud; cooperators acted as though one might blossom.
Why would you choose to cooperate or cheat? The answer hinges, essentially, on a guess: For many encounters, you simply can't know whether or not they'll be one-shot or repeated. "Given the stochastic nature of the world," the authors write, "it might be correct to say that, at the time of the interaction, the interaction is not determinately either one-shot or repeated." You can make an educated guess, but a certain amount of uncertainty and randomness is unavoidable. Cut in line at the movies, and you can never be totally sure that the guy behind you won't be interviewing you for a job.
Tooby and Cosmides ran their simulation for tens of thousands of generations, to figure out where the generosity thermostat would get set. They find that it makes more sense to adopt a general attitude of generosity, in the hope that paying it forward now will pay back later. What does this all mean for how we think about ourselves? To the researchers, it suggests that "human generosity, far from being a thin veneer of cultural conditioning atop a Machiavellian core, may turn out to be a bedrock feature of human nature." Why? Because thousands of years of small-town living have left their mark.
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