The custom or law in all societies prohibiting sex and marriage between members of the nuclear family, the universality of the taboo being notoriously difficult to explain. According to the functional theory, the taboo creates and maintains networks of social relationships without which societies disintegrate into separate nuclear families and are eliminated, but this theory rests on the controversial assumption that without the incest taboo people would naturally want to mate with close relatives, and it fails to explain why the taboo governs sexual intercourse rather than marriage. According to the inbreeding depression theory, inbreeding leads to the genetic deterioration and ultimate extinction of any society that practises it, because harmful recessive genes are more likely to come together, and the taboo may thus have evolved by natural selection, but this leaves open the question as to why few non-human animals have incest taboos and why some successful human societies, such as the Sotho people of southern Africa, have a positive obligation to marry cousins. The most persuasive explanation is the prepubertal interaction theory, put forward in 1894 by the Finnish social philosopher Edvard Alexander Westermarck (1862–1939), based on the psychological observation that intimate contact between children at a critical period before puberty, now believed to be between 2 and 6 years of age, results in a lack of sexual attraction between them in adult life. See also adelphogamy (2), inbreeding depression.