The origins of altruism in offspring care.

The current review aims to unify existing views of altruism through an examination of the biological bases of a fundamental form of giving: altruistic responding. Altruistic responding is most salient during heroic acts of helping but is also observed any time one perceives another's distress or need, which in turn motivates one to help at a current cost to the self. Such aid is simple, observable across species, and rooted in the instincts and circuits that evolved to maximize inclusive fitness through the care of helpless offspring. By design, the system already biases aid to both kin and nonkin under conditions that are largely adaptive. These inherent benefits are also buttressed in primates and humans by known, later-arriving benefits to helping in group-living animals. Evidence for the proposed homology between altruistic responding and offspring retrieval is presented through 10 key shared factors. Conceptually, both require (a) participation by nonmothers, (b) motor competence and expertise, (c) an adaptive opponency between avoidance and approach, and a facilitating role of (d) neonatal vulnerability, (e) salient distress, and (f) rewarding close contact. Physiologically, they also share neurohormonal support from (g) oxytocin, (h) the domain-general mesolimbocortical system, (i) the cingulate cortex, and (j) the orbitofrontal cortex. The framework intermixes ultimate and proximate levels of analysis and unifies existing views by assuming that even complex human behaviors reflect ancient mammalian neural systems that evolved to solve key problems in adaptive ways, with far-reaching consequences for even our most venerated human traits.