A neo-Darwinian heuristic for decisions involving altruism is hypothesized in this article. Following W. Hamilton’s (1964) analysis of inclusive fitness, the assumption of this study is that tendencies to help another person are selected against, except when the donor and recipient are related and share genes underlying these tendencies. An important social psychological implication of Hamilton’s formulation is that in group-living individuals (1) natural selection favors those who are prone to help others as a function of the latters’ relatedness, potential fecundity, or other features indicating a recipient’s capacity to enhance the donors’ inclusive fitness, and (2) this effect is especially strong when help is biologically significant. Such a heuristic is demonstrated in several studies involving hypothetical decisions to help: In life-or-death situations, people chose to aid close kin over distant kin, the young over the old, the healthy over the sick, the wealthy over the poor, and the premenopausal woman over the postmenopausal woman; whereas when it is a matter of an everyday favor, they gave less weight to kinship and opted to help either the very young or the very old over those of intermediate age, the sick over the healthy, and the poor over the wealthy. (PsycINFO Database Record (c) 2012 APA, all rights reserved)