Evolutionary theory has straightforward relevance to parental behavior. The behavioral inclinations that natural selection favors are those that contribute to Darwinian fitness, that is, to one’s expected genetic posterity (in the statistical, not the psychological, sense of “expected”). The primary avenue by which people and other creatures promote their fitness is by producing viable young who will eventually reproduce. Parental motives, emotions, and actions are therefore prime targets of selection.

“Parental investment” (Trivers, 1972) is a limited resource that parents have evolved to allocate in ways that can be expected to maximize the eventual reproductive success of one’s total progeny (Clutton-Brock, 1991; Royle, Smiseth, & Kolliker, 2012). This means investing preferentially in young whose individual attributes predict that the investment will be most beneficial, but above all, it means investing preferentially in one’s own young (Daly & Wilson, 1980). Why a Darwinian would predict that parents will avoid squandering their limited resources on unrelated young should be obvious: selection favors those genes and traits that enhance their carriers’ fitness relative to the fitness of conspecific rivals.