One of the most common taboos across human societies of the past and present has been incest. Virtually every known culture has considered it repulsive, especially when involving siblings or a parent and child. The leading behavioural theory that has been proposed to account for the ubiquity of this aversion is known as the Westermarck effect, after Finnish scholar Edvard Westermarck, who proposed it in his 1891 book The History of Human Marriage. The idea of the Westermarck effect is that young children will become sexually/romantically desensitised to anyone they live in close contact with over the course of the first few years of their lives. That is, they will reach adulthood with no compulsion to consider a relationship with anyone they shared a home with in their early childhood. Note that crucially, the connection does not have to be biological; according to the theory, it applies just as readily to children adopted at a young age as to those raised by their birth parents. But since children are likely to be raised by at least one of their biological parents – about 97.5% of children in the U.S., according to the 2000 census – the effect is thought to have arisen through evolution because it reduces the chances of inbreeding, which can tie the gene-pool up in ugly knots of emergent recessive traits. It functions well in this respect. However, when a child is separated from biological family at an early age, there is no chance for the Westermarck effect to take hold; reunions between biological relatives who were separated much earlier sometimes lead into unforeseen emotional territory.

The Westermarck effect is a hypothesis, but there is evidence to support it. Some Israeli citizens live in communal homesteads named kibbutzes (or kibbutzim in Hebrew). Property is usually shared, income is often doled out more or less equally, and children are all raised together in groups according to age. Unsurprisingly, the kibbutz model has been of major interest to sociologists, anthropologists, and psychologists. The finding relevant to the Westermarck effect is that young adults in the same age-group are seldom attracted to each other, even when their parents more or less expect them to be. A study by American cultural anthropologist Melford Spiro that examined 3,000 marriages within the kibbutz system found that only about 15 weddings involved pairs of people who were raised in the same group of children. Furthermore, none of these pairs had been raised with their partners before the age of six. This strongly suggests a sort of ‘critical period’ for the Westermarck effect, operating behind-the-scenes for the first six years of life.

I really liked your topic. Thank you very much for the information.