Incest, like the whalebone corset and monarchy, has mostly gone out of fashion (though it might be getting a bit of a boost lately with the incestuous Lannisters on "Game of Thrones").
With very strong cultural and instinctual forces underlying the human taboo against incest, you might be prompted to ask why it ever happened in the first place.
Nature tends to abhor keeping it in the family, and with good reason. Swapping genes with someone whose DNA is fairly similar to your own increases the chance that, if you carry some rare genetic mutation normally masked by a dominant “wild-type” version of the gene, your offspring will have two copies of that recessive allele. When reproducing with close relatives becomes the norm, the odds of hitting the genetic-disorder jackpot become higher.
An abnormally high rate of deformities found in early human fossils discovered in China suggests that at least in some regions, inbreeding was common. Whether the practice was cultural, or out of necessity, or some combination of the two, remains to be seen. Our ancestors may have lived in isolated areas where it was easier to keep it in the family rather than go searching abroad for a mate.
Whether it happens for reasons of convenience or pedigree, marrying a close relative tends to result in severe health effects. Witness the incidence of hemophilia in European royal families of the 19th and 20th centuries or the frequent occurrence of genetic disorders like Tay-Sachs disease and cystic fibrosis in isolated ethnic or religious enclaves, such as the Ashkenazi Jews or various Amish communities. The effect can also be seen in certain endangered species – one of the more pressing concerns with cheetahs and rhinos is saving enough of them to maintain a stable, healthy population.
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To combat the convenience of incest, many animals and plants carry a host of natural mechanisms that guard against inbreeding. One of the more common methods is dispersal – movement of a creature from its place of birth. Another possible method, especially among socially monogamous species, is promiscuity.
Take the case of splendid fairy-wrens, which live in small groups consisting of one ostensibly monogamous breeding pair, along with several helper birds of either sex that tend to be closely related to the main pair. Thanks to the close social ties of the group, if all offspring were sired by the senior male, there would be an extremely high rate of inbreeding; but fairy-wrens, it turns out, tend to stray. One study found that nearly two-thirds of the offspring of 24 females were fathered by a male that was not in their particular social group. In a 2011 study of red-winged fairy-wrens, 70 percent of the offspring resulted from adulterous mating.
Flowering plants, which commonly have both “male” and “female” organs, have a variety of ways to stop self-fertilization. If a plant’s pollen grain lands on its own stigma or on a stigma of a very close relative, its movement towards the egg will be arrested by any one of several molecular mechanisms.
For humans, we can add a layer of psychology to the mechanisms of incest avoidance. The hypothetical Westermarck effect posits that people become naturally sexually desensitized to those they have cohabitated with from an early age – basically, that it’s natural to be grossed out by the idea of kissing your sibling.
Modern humans seem to be opting more and more for the dispersal method. Centuries ago, it was difficult for most of humanity to dream of traveling 100 miles from their hometown. Now, thanks to a flatter modern world, it’s not as unusual that the U.S. president is the son of a woman born in Kansas and a man from Kenya.
But even with air travel and psychoanalysis, one can never be sure. Humans have been around and breeding only for a comparatively short time, so you might be more closely related to an attractive stranger than you think.
“Taking all family ties into account, the person you sat next to on the bus this morning is, on average, likely to be something like your sixth cousin, which means that the two of you probably share at least one ancestor from the time of the Paris Commune” (1871), geneticist Steve Jones wrote for The Telegraph on Tuesday.