Inbreeding hurts the next generation’s reproductive success | Science News


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Inbreeding hurts the next generation’s reproductive success

Preliminary data tally birth stats of kids with closely related parents

5:46pm, October 23, 2017
Charles II

KIN’S KING  Charles II of Spain was the last Habsburg ruler of Spain. His distinctive jawline, infertility and mental disabilities may have been the product of the royal family’s extreme inbreeding.

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ORLANDO, Fla. — Kissing cousins aren’t doing their children any evolutionary favors, some preliminary data suggest.

Mating with a close relative, known as inbreeding, reduces nonhuman animals’ evolutionary fitness — measured by the ability to produce offspring. Inbreeding, it turns out, also puts a hit on humans’ reproductive success, David Clark of the University of Edinburgh reported October 20 at the annual meeting of the American Society of Human Genetics.

Offspring of second cousins or closer relatives make up about 10 percent of the world population, Clark said. He and colleagues collected data on more than a million people from more than 100 culturally diverse populations and calculated the effect inbreeding has on traits related to evolutionary fitness.

Compared with outbred peers, offspring of first cousins have 1.4 fewer opposite-sex sexual partners, have sex for the first time 11 months later, have 0.11 fewer children and are 1.6 times as likely to be childless — all indicators of reduced reproductive ability. Childlessness was not because of a lack of opportunity to have kids, but rather because of fertility problems, Clark said. Children of first cousins are also 1 centimeter shorter, on average, than their peers and 0.84 kilograms lighter at birth. They also have five fewer months of education, presumably because they have less intellectual capacity than people with more distantly related parents, Clark said.

The more closely related the parents, the bigger the hit on reproductive fitness. Children of incest are 3 centimeters shorter and four times as likely to be childless than outbred peers, Clark said.


D.W. Clark et al. Sex-specific inbreeding depression in humans. American Society of Human Genetics, Orlando, Fla., October 20, 2017.

Further Reading

B. Bower. Ancient humans avoided inbreeding by networking. Science News. Vol. 192, October 28, 2017, p.7.

T.H. Saey. Gene knockouts in people provide drug safety, effectiveness clues. Science News. Vol. 191, May 13, 2017, p. 10.

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