The Westermarck Effect is the hypothesised psychological effect that people who live in close proximity during the first few years of their lives become desensitised to sexual attraction. That is, that people who are close when they are babies/children don’t want to bang each other when they grow up.
It was first hypothesised by a guy called Edvard Westermarck in his book The History of Human Marriage (weird title because as far as I’m aware human marriage is the only type of marriage available), and the effect is sometimes referred to instead as “reverse sexual imprinting” or "negative imprinting".
The effect would usually apply to siblings and so could function as a defence against inbreeding, but it would also carry across to other individuals who a person may be raised around.
The effect is in direct contradiction to Freud’s ideas about familial sexual attraction and the Oedipus complex.
There are some studies and many observations that appear to support the concept of the Westermarck effect, but it also has been argued against. Specifically, in that rather than it being an innate psychological effect it may instead be a cultural phenomenon.
Interestingly the opposite effect appears to be true for biological relatives who are separated and raised apart, through a phenomenon named genetic sexual attraction.
Marcinkowska, U. M., Moore, F. R., & Rantala, M. J. (2013). An experimental test of the Westermarck effect: sex differences in inbreeding avoidance.
Rantala, M. J., & Marcinkowska, U. M. (2011). The role of sexual imprinting and the Westermarck effect in mate choice in humans.
Walter, A., & Buyske, S. (2003). The Westermarck Effect and early childhood co‐socialization: Sex differences in inbreeding‐avoidance.