9.6 Incest Avoidance

While not inherently dangerous, interbreeding between closely related individuals does carry higher than average risk for genetic diseases. This is managed in chimpanzees and bonobos by young adults leaving their natal group when after entering their reproductive years. How do humans handle incest avoidance?

Much like our favorite brother and sister from Game of Thrones, royalty and dynastic intercourse was often kept in close relation. This frequently tied to cultural mythos derived from the belief (or story) that the royal family were descendants of the gods. For example, many early Roman and Egyptian relationships occurred between siblings, half siblings, and first cousins.

In fact, Charles Darwin himself was married to his cousin, Emma Wedgwood – a fact he began to fret about as his understanding of evolution formed. Born with a “weak constitution,” Darwin was afraid he may pass health concerns to his children. Interestingly, this entered his writings in The Variation of Animals and Plants under Domestication, where he praised the value of outcrossing as beneficial for plants and animals.

There are two main studies to suggest that incest avoidance may actually be part of our primate ancestry.

Westermarck Effect

In the late 1800s, a man by the name of Edvard Westermarck began to hypothesize that something other than culture served as a barrier to these closely related pairings. Later deemed “The Westermarck Effect,” his hypothesis argued that biological relatedness had less influence on incest avoidance than developmental histories. That is, adults were significantly less likely to be sexually attracted to people with whom they grew up. His hypothesis detailed the following:

1. Inbreeding is detrimental to offspring health
2. Knowing peers early in life results in an aversion to sexual behavior with those peers
3. This aversion is likely an evolutionary adaptation meant to reduce the risk of inbreeding
4. This aversion also manifests within cultures as the “incest taboo”

Supporting Westermarck’s hypothesis, Arthur Wolf conducted studies of Taiwanese minor marriages. In 1900-1925 Taiwan, minor marriages were the result of adoption. Couples would adopt a young girl and raise her to join the family as a future daughter-in-law. Once both son and adopted daughter were of marriageable age, they would be wed with all the expectations of family that come with marriage.

Despite the view that incest avoidance was driven by biological relatedness, the Taiwanese minor marriages supported Westermarck’s hypothesis. Approximately 1/5th of these marriages ended in divorce, and another 1/5th resulted in the bride’s adultery. Most importantly, these marriages averaged less than one child over a 5-year period. This means that many of the couples were not having any children, likely as the result of unconsummated marriages. Sadly, there are also reports of frequent suicide on the part of the bride, and grooms who ran away within the first years of marriage.