The Westermarck effect is a phenomenon which has been observed in individuals who spend large amounts of time with each other under the age of six. People who are raised together, regardless of relationship, tend to become desensitized to each other, and they will not generally develop sexual attraction to each other later in life. A variety of studies have supported the concept of the Westermarck effect.
This idea is sometimes referred to as “reverse imprinting,” and it is named for Edvard Westermarck, a Finnish sociologist who worked and wrote in the late 1800s. He was particularly interested in marriage patterns and incest taboos, and his idea that people who are raised together will not develop sexual attraction went contradictory to the beliefs of Freud, a prominent contemporary. Over time, it has been apparent that Westermarck was vindicated, as evidence strongly suggests that Freud's ideas are not supported by actual evidence.
In addition to using data about brothers and sisters who are raised together, researchers on the Westermarck effect have also looked at situations in which non-related individuals are raised together. For example, on Israeli kibbutzim, children are often raised together in large peer groups, and members of the same peer group rarely develop relationships of a sexual nature with each other. This also holds true for young children adopted into households with existing children.
In contrast, siblings who are raised apart sometimes develop a sexual attraction to each other when they meet later in life, developing what is known as genetic sexual attraction. Researchers on the Westermarck effect have also found that the six year old cutoff is very important; children who are raised together after the age of six do not demonstrate the Westermarck effect, indicating that it has to do with early childhood development.
Opponents of this theory often point to historical examples of sibling marriages, such as those performed in Ancient Egypt among the ruling classes. However, these marriages are not a good counterexample, because such marriages were typically contracted without consulting those involved, and it was common for children of the ruling classes to be raised separately from each other, for a variety of reasons.
In an interesting modern example of the Westermarck effect at work, researchers studied traditional Chinese families, who sometimes adopt a young girl into their households with the intention of marrying the girl to their sons. They discovered that the girls are often strongly opposed to such marriages when they come of age, and that these marriages are more prone to later dissolution, childlessness, or adultery.