Altruism among relatives and non-relatives.

Hamilton's [Hamilton, W.D., 1964. The genetical evolution of social behavior, I, II. J. Theor. Biol. 7, 1-52] kin-selection theory predicts that altruism will be greater with greater genetic overlap (degree of kinship) between giver and receiver. Kin may be identified in terms of social distance-the closer you feel to someone else, (a) the greater your genetic overlap with them should be, and (b) the more altruistic you should be toward them. The present experiment determined the amount of their own (hypothetical) monetary reward undergraduates were willing to forgo in order to give $75 to other people at various social distances. We found that (a) genetic relationship and (b) altruism varied inversely with social distance; the closer you feel to someone else, the closer their relation to you is likely to be, and the more altruistic you are likely to be toward them. However, even at the same social distance, participants were willing to forgo significantly more money for the benefit of relatives than for the benefit of non-relatives. These results are consistent with kin-selection theory and imply that altruism is determined by factors in addition to social distance.