Primate Mother-Son “Incest”

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Abstract
The reports that primates almost never mount or copulate with their mothers now appear to be incorrect. Young males and females have occasionally been observed mounting their mothers; males sometimes intromit and thrust. There are good reasons to believe that such mother-son sexual activity has some function other than reproduction. (1) The males involved are either immature or adolescent; fully mature males almost never mount their mothers. (2) Sexual encounters of mother and son differ markedly from normal mating behavior. (3) The male or female offspring is most likely to mount when upset. (4) The mother is frequently, perhaps usually, anestrus when mounted by the son. Mounting of the mother may serve two functions, the reduction of stress and the same ontogenetic function as the mounting of immature peers. Whether primate consanguineous mating is common or rare is relevant to the furious debate about the incidence of human incest and whether nuclear family incest barriers are a function of genetic predisposition as well as cultural prohibitions. That copulation of primate mother and son is infrequent and occurs almost always while the son is young strengthens the belief that incest barriers of the human nuclear family are genetic as well as cultural.