The Westermarck effect, also known as reverse sexual imprinting, is a psychological hypothesis that states that people tend not to be attracted to peers with whom they lived like siblings before the age of six. This hypothesis was first proposed by Finnish anthropologist Edvard Westermarck in his book The History of Human Marriage (1891) as one explanation for the incest taboo.

In psychology and ethology, imprinting is any kind of phase-sensitive learning that is rapid and apparently independent of the consequences of behaviour. It was first used to describe situations in which an animal or person learns the characteristics of some stimulus, which is therefore said to be "imprinted" onto the subject. Imprinting is hypothesized to have a critical period.