Natural selection does not only select for physical traits — it also shapes behavior. Behaviors that increased survival and reproduction in ancestral environments were more likely to be inherited by future generations. Today's human psychology — our social instincts, our fears, our preferences — reflects millions of years of selection pressures. Evolutionary psychology studies how these ancestral adaptations shape modern behavior, from mate choice to moral intuitions.

Hamilton's rule explains the evolution of altruism toward genetic relatives: an organism benefits from helping relatives in proportion to their genetic similarity. You share half your genes with a sibling, a quarter with a cousin. This predicts the observed pattern that organisms invest more in closer relatives. This "kin selection" explains why parents sacrifice for children, why social insects like bees maintain non-reproductive workers, and why nepotism is a universal human tendency.

Not all cooperation requires genetic relatedness. Reciprocal altruism (Trivers 1971) explains cooperation between non-relatives: help now, expect help later. This requires memory, recognition of individuals, and mechanisms to punish cheaters. Computer simulations (Axelrod's tournaments) showed that "tit-for-tat" strategies — cooperate first, then mirror the other's last move — are highly successful in iterated games. This may explain much of human social cooperation: trade, alliance, friendship, the expectation of reciprocity that underlies everyday social life.