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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.

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. That is, when a pair of biologically related individuals meet for the first time in adulthood, they often find each other very attractive. 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).

GSA is not inevitable, but it is common. The term was coined by American Barbara Gonyo. Pregnant at 15 in the mid-1950s, Gonyo was forced to give her son Mitch up for adoption when he was born. The two found each other again around 1980, and Gonyo, then 42, was horrified to realise that she was feeling very attracted to her 26-year-old son. Even allowing for Mitch’s resemblance to his father, Gonyo’s first love, Gonyo’s reaction struck her as extreme and disgusting. Eventually, though, she came to terms with her feelings, attributing them to the lack of bonding in her son’s early childhood. Fortunately, her son did not reciprocate, and they did not pursue a relationship.

But in some cases of genetic sexual attraction, the feelings are mutual. A Canadian woman identified as ‘Sally’ and her biological son felt immediately attracted to one another upon meeting again, 30 years after the boy’s birth. Their physical relationship developed, and the young man could hardly believe that the woman he found to be a perfect match was his biological mother. American couple Rachel and Shawn met in 1999 when they were both 27 and have been an enthusiastically happy couple since. They sought each other out after learning that they shared a birth father. Rachel and Shawn – who are engaged but legally prohibited from getting married – discussed in a 2007 report with ABC News all the ways in which they are a perfectly ordinary couple. They are even devout Christians; but their deep love for one another is what they consider the most important part of their lives.

It is reported again and again. Genetic sexual attraction – both one-sided and reciprocal – has been known to take hold between reunited siblings, a parent and child, an uncle and niece, and more. Typically the attachment is so strong that the individuals involved are compelled to overlook all else.

Jennifer and John, half-siblings from England, left their respective spouses and moved in with one another. American Kathryn Harrison was seduced by the biological father she met in adulthood and carried on a relationship with him for four years. A pair of sisters, neither of whom reported any prior attraction to women, fell in love with each other. Americans Phil and Pearl became highly attracted to one another after meeting; Pearl is Phil’s biological grandmother, who gave Phil’s mother up for adoption after giving birth to her at the age of 18. Germans Patrick and Susan – biological siblings who did not meet until adulthood – fell in love, have given birth to four children (at least two of whom are developmentally disabled), and have been fighting German incest laws ever since.

Few scientific investigations of the phenomenon known as GSA exist, but one was conducted by Maurice Greenberg and Roland Littlewood of University College London. The researchers looked at more than 40 cases of GSA and ended up concluding that up to 50% of reunions between adults who had been separated by early adoption involve GSA on the part of at least one of the individuals. In other words, what looks like taboo most of the time appears to be a fairly ordinary response to the exceptional circumstances of biological family members being brought back together long after parting. Greenberg notes that the connections established between reunited family members tend to be profound and respectful, and nothing at all like cases of incest involving non-consenting individuals. He also found evidence for the Westermarck effect operating elsewhere in his participants’ lives: when Greenberg asked individuals affected by GSA whether they would ever consider forming romantic/sexual relationships with members of their adopted families, they tended to shudder with repulsion.

One major question raised by any examination of sexual relationships involving biological family members is where, exactly, this leaves Sigmund Freud. Freud’s pioneering work in psychoanalysis has contributed an immense amount to the modern field of psychology, but he has become notorious for having proposed that infants are sexually attracted to the parent of the opposite sex and that these feelings have to be suppressed – almost the diametric opposite of the Westermarck effect theory. While there is some evidence that heterosexual men and women may favour potential partners who share physical features with their opposite-sex parents, for most of us the thought of sexual attraction to a parent is absurd. 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.”

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