The Westermarck Effect minimizes the risk of incest

Some interesting observations and theory, summarized at Damn Interesting:

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...

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... 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... 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...

Another source of evidence for the Westermarck effect comes from what happens when it is noticeably absent. Genetically related individuals who are not raised together often fail to be sexually and romantically blind to each other....Genes ensure that the two have a lot in common, and the absence of the Westermarck effect sometimes makes them difficult for one another to resist. This is a converse theory known as genetic sexual attraction (GSA)....

Freud did not think much of Westermarck’s ideas, and he himself actively recalled having once had a fairly noticeable physical reaction to his naked mother getting dressed. But, says cognitive scientist Steven Pinker in his book How the Mind Works, it is Westermarck who may have the last word. Pinker speculates that the reason why Freud was able to have such a dramatic response to the sight of his own mother putting her clothes on was that as a baby, Sigmund had been cared for by a wet nurse. Perhaps it was this nurse, and not his mother, to whom Sigmund became desensitised; in other words, the Westermarck effect was not fully activated between the young Freud and his biological mother. “The Westermarck theory,” Pinker says in summary, “has out-Freuded Freud.”... 

More at the Damn Interesting, via Neatorama.