From The Chronicle of Higher Education" For a fairness instinct to have evolved by natural selection, those who possessed it must have been disproportionately successful in projecting their genes for it into the future. How might that have happened? Wouldn't each individual be more fit taking whatever he or she could get, rather than turning down opportunities simply because they were unfair and thereby sometimes getting nothing at all? Why bother yourself with what others are getting—i.e., with fairness—instead of just trying to maximize your own payoff?
There are several possible answers. For one, consider that in some circumstances, one's payoff is very much a function of what others are getting ... and giving. That is particularly true of so-called reciprocal altruism, misnamed because it isn't based on altruism at all, but rather on a selfish exchange whereby the donor is subsequently repaid (and then some) by the recipient.
Another biological basis for fairness probably derives from the fact that natural selection doesn't operate by absolute reproductive success but by relative measures. Fitness is most meaningfully described as a fraction, with the numerator being the abundance of any gene(s) in question and the denominator being that of all alternative forms of the same gene. Thus it isn't simply a matter of how successful you are, but how you stack up relative to others. As in the joke, "How's your husband?" Answer: "Compared to what?" The key evolutionary question isn't "How am I doing?" Rather it's "How am I doing compared to everyone else?"