Infanticide

Infanticide occurs in all human societies, usually as a sub-category of homicide (i.e. societally proscribed killing of conspecifics), but it also is prescribed in some human societies, e.g. after the birth of twins. Distinctions are drawn between killing the offspring of outsiders (e.g. xenophobia), non-related group members (e.g. illegitimacy), or own offspring (e.g. birth defects). Killing of neonates may be required, while killing of older infants is forbidden. In apes, two species, chimpanzee and gorilla, show both inter- and intra-group infanticide, of a wide age-range of infants, from newborns to weanlings. Two other species, bonobo and orang-utan, have never been seen in nature to kill infants, although in released, former-captive orang-utans, cannibalism of own infants has been recorded. In other non-human primates, infanticide has been reported in a wide range of species, from lemurs to macaques, usually committed by adult males.

Intense debate on the frequency and normality of infanticide in non-human primates once raged, with explanatory hypotheses ranging from adaptive (e.g. sexual selection), accidental by-product (e.g. of aggression) to pathological (e.g. from stress of crowding). Current opinion favours Hrdy’s sexual selection hypothesis, that is, as a male strategy to bring females back into reproductive cycling versus female counter-strategies of resistance (e.g. pseudo-estrus). Infanticide rates in humans vary predictably according to certain independent variables, e.g. presence of a step-father.

The relationship between cannibalism and infanticide are perplexing: All known cases of cannibalism by non-human primates in nature are of infants; most cases of infanticide in nature do not lead to cannibalism. Definition of infanticide varies in human societies, e.g. termination of pregnancy may be included (for fetuses) or not (for embryos).