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, one explanation for the incest taboo, 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.
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. This supports the theory that the populations exhibiting the Westermarck effect became predominant because of the deleterious effects of inbreeding on those that didn't.
Westermarck and Freud 
Freud argued that as children, members of the same family naturally lust for one another (See Oedipus complex), making it necessary for societies to create incest taboos, 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
Sociologists and anthropologists have criticized the validity of research presented in support of the Westermarck effect and the contention that it serves as an ultimate demonstration for the viability of natural selection theory in explaining human behaviour. For example, a 2009 study by Eran Shor and Dalit Simchai demonstrated that although most peers who grew up closely together in the Israeli kibbutzim did not marry one another, they did report substantial attraction to co-reared peers. The authors conclude that the case of the kibbutzim actually provides little support for the Westermarck Effect and that childhood proximity cannot in itself produce sexual avoidance without the existence of social pressures and norms.
- Shepher, Joseph (1983). Incest: A Biosocial View. Studies in anthropology. New York: Academic Press. ISBN 0-12-639460-1. LCCN 81006552.
- Freud, S. (1913) Totem and Taboo in The Standard edition of the Complete Psychological works of Sigmund Freud, Vol XIII
- Shor, Eran; Simchai, Dalit (2009). "Incest Avoidance, the Incest Taboo, and Social Cohesion: Revisiting Westermarck and the Case of the Israeli Kibbutzim". American Journal of Sociology 114 (6): 1803–1842.
- Paul, Robert A. (1988). Psychoanalysis and the Propinquity Theory of Incest Avoidance. The Journal of Psychohistory 3 (Vol. 15), 255–261.
- Spain, David H. (1987). The Westermarck–Freud Incest-Theory Debate: An Evaluation and Reformation. Current Anthropology 5 (Vol. 28), 623–635, 643–645.
- Westermarck, Edvard A. (1921). The history of human marriage, 5th edn. London: Macmillan.
- Lieberman, D., Tooby, J. & Cosmides, L. (2007). The architecture of human kin detection, Nature, 445, 727-731