Free An Introduction to Evolutionary Ethics by Scott M. James

a figure of speech; for some primates and monkeys it's a serious request. A vervet monkey, for example, has to deal constantly with external parasites, some of which can cost him his life. But he can't reach all the parts of his body that might be vulnerable (ever tried putting sunscreen on the middle of your back?). So he needs a groomer , another monkey who will spend thirty minutes or so carefully picking out parasites from his head and back. Thirty minutes might not seem like a lot of time, but it's time that could be spent hunting or foraging, attracting potential mates or caring for young – in other words, advancing his own reproductive fitness. If grooming behavior occurred strictly within the family, then one need only appeal to the processes of inclusive fitness. But biologists routinely observe monkeys grooming non-relatives . Why? As in the case of vampire bats, something else is going on here. What's going on, according to biologists, is reciprocal altruism. In study after study (most recently, Schino 2007), primatologists observe that whether one monkey (P) grooms another monkey (Q) now is directly related to whether Q has groomed P in the past. Moreover, the length of time spent grooming is proportional to the time spent in past exchanges.
    In another study, anthropologist Craig Packer (1977) showed that whether or not a vervet monkey (R) was disposed to assist an unrelated monkey (S) calling out for help is directly related to whether S had groomed R in the recent past. If S had recently groomed R, R was far more likely to look around and move in the direction of S's calls of distress. By contrast, if S had not recently groomed R, R simply ignored the calls. (Interestingly, this discrepancy does not appear among kin; there, calls for help are responded to whether or not grooming has taken place.) So it appears that vervet monkeys are “keeping score.” And for good reason: doing favors for one's neighbors pays. Equally important are the costs. With the exception of a few dominant individuals at the top of the social hierarchy, vervet monkeys that do not return the “grooming favor” significantly increase their chances of contracting a disease.
    As a general rule, then, mutual cooperation is better for everyone involved than mutual defection . Mutual cooperation, we'll say, consists of individuals benefiting others in return for some future benefit. Mutual defection consists of individuals refusing to benefit others in return for some future benefit. If Farmer A and Farmer B aren't willing to assist each other, then Farmer A and Farmer B face desperate futures. Clearly, mutual cooperation is a far better alternative. But this is not the end of the matter.
    Although mutual cooperation yields higher returns for everyone than mutual defection, any individual stands to gain even more under a different arrangement: she defects while others cooperate. This is the fabled free-rider . If Farmer B helps Farmer A harvest the latter's crops but Farmer A does not return the favor, then Farmer A has received a sizable benefit without having to pay the cost (of returning the favor). If, that is, this were a one-time affair (because, let's say, Farmer A immediately packed up his harvest and moved to the other side of the continent), then we'd have to say that Farmer A did better under this arrangement than under mutual cooperation. As far as Farmer B is concerned, this arrangement is even worse than mutual defection since he made a sizable sacrifice on Farmer A's behalf but received nothing in return. We can thus add two more general rules to our list. First, the best arrangement for any individual is one in which she defects (i.e., receives help, but doesn't help others) while others cooperate. Second, the worst arrangement for any individual is one in which she cooperates while others defect.
    Perhaps the best way to appreciate the intricacies of reciprocal exchanges is by considering the game “Prisoner's Dilemma,”