The Westermarck effect, or reverse sexual imprinting, is a hypothetical psychological effect through which people who live in close domestic proximity during the first few years of their lives become desensitized to later sexual attraction. This phenomenon was first hypothesized by Finnish anthropologist Edvard Westermarck in his book The History of Human Marriage (1891). Observations interpreted as evidence for the Westermarck effect have since been made in many places and cultures, including in the Israeli kibbutz system, and the Chinese Shim-pua marriage customs, as well as in biological-related families.
In the case of the Israeli kibbutzim (collective farms), children were reared somewhat communally in peer groups, based on age, not biological relation. A study of the marriage patterns of these children later in life revealed that out of the nearly 3,000 marriages that occurred across the kibbutz system, only fourteen were between children from the same peer group. Of those fourteen, none had been reared together during the first six years of life. This result suggests that the Westermarck effect operates during the period from birth to the age of six.[1]
When proximity during this critical period does not occur — for example, where a brother and sister are brought up separately, never meeting one another — they may find one another highly sexually attractive when they meet as adults, according to the hypothesis of genetic sexual attraction (q.v.). This supports the claim that the Westermarck effect evolved because it suppressed inbreeding.
Freud argued that as children, members of the same family naturally lust for one another, making it necessary for societies to create incest taboos,[2] but Westermarck argued the reverse, that the taboos themselves arise naturally as products of innate attitudes.
Steven Pinker wrote on the subject:
The idea that boys want to sleep with their mothers strikes most men as the silliest thing they have ever heard. Obviously, it did not seem so to Freud, who wrote that as a boy he once had an erotic reaction to watching his mother dressing. But Freud had a wet-nurse, and may not have experienced the early intimacy that would have tipped off his perceptual system that Mrs. Freud was his mother. The Westermarck theory has out-Freuded Freud.—Steven Pinker, How the Mind Works