The Westermarck Effect

In the late 1800s, Edward Westermarck was sitting around and thinking about the incest taboo.

Across the world and throughout history there have been strict taboos against sexual relationships between close blood relatives (with a few notable and creepy exceptions – most famously among royals in Ancient Egypt: Cleopatra, for example, married her brother).  While the specific definition of “close” varies from culture to culture and time to time, sexual or marital relationships between siblings or between parents and children are almost universally prohibited.

When Edward (or Edvard) Westermarck was doing his thinking, it was the opinion of many learned men that the incest taboo was a societal construction.  Sir James Frazier, one of Westermarck’s critics, wrote of the incest taboo that “the law only forbids men to do what their instincts incline them to do”, which perhaps says more about Sir Frazier than about men in general.

Westermarck, however, apparently didn’t want to have sex with his close relatives, and didn’t believe that the incest taboo was a top-down cultural regulation.  He noticed that most people, rather than longing desperately to have sex with their relatives and sitting on their hands for the sake of the law and the neighbors, felt a profound and visceral disgust at the idea of incest.

Westermarck hypothesized that there might be a critical period during childhood development, and the people to whom a child is heavily exposed during that critical period will never be sexually attractive to them.

With the shadow of Freud looming over everyone and insisting that it was completely normal to want to have sex with your mother, psychologists and biologists cheerfully ignored Westermarck for decades.

In the past fifty years, however, scientists have gone looking for evidence of the Westermarck effect (which is also called ‘reverse sexual imprinting’).  The first evidence came from observations of several species of macaque monkeys.  Scientists (Sade most explicitly, in 1968) discovered that, despite the Freudians, monkey sons don’t have sex with their mothers.  When male animals win their way to the alpha spot in their troop, they take sexual advantage of their new position, and mate with all the females in the group, except their mothers.  Even in animals groups where sexual pairings are relatively indiscriminate, males sexually avoid their immediate blood relatives.

Only among scientists, who had to retreat to the lab to rediscover something as basic as maternal love, would the information that sons don’t want to have sex with their mothers come as breaking news.  But they finally discovered it, after all, several decades too late for poor Westermarck, who died in 1939.

Featured image is a bronze coin of Cleopatra from the collection of the British Museum.

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