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Sunday, January 25, 2015

"Why Marrying Your Cousin May Pay Off"

Oh my god. This is the strongest academic evidence I've seen to date. This is the kind of research I've only dreamed about. My jaw is on the floor.
In traditional residential societies, couples who were more closely related to each other had more children. By contrast, in migrant societies, related spouses had fewer direct descendants, the research revealed.
[...] "There's this counterintuitive finding that higher spousal relatedness is related to higher reproductive success in several humans societies," said Drew Bailey, a psychologist at Carnegie Mellon University in Pittsburgh, and co-author of the study detailed May 21 in the journal Biology Letters. Even in modern, industrialized society, research suggests that people tend to marry others with similar DNA.
[...] In the new study, Bailey and his colleagues examined 46 small-scale societies to compare the effect of inbreeding on the fitness of foragers and non-foragers. The non-foraging societies included horticulturalists, agriculturalists and herding populations, whereas foraging societies were primarily hunter-gatherers. The researchers defined fitness as the number of surviving children in the family tree.
In line with previous findings, the researchers found that among non-foraging societies, a couple's relatedness was linked with having more surviving children. But among foraging societies, the opposite was true: More-closely related spouses had fewer surviving children. Furthermore, the more family intermarriage in a society, the greater the benefit of intermarrying on the number of children couples had. In other words, in societies in which people frequently married their relatives, intermarrying showed a stronger link to having more children.
[...] There could be many explanations for the different effects of inbreeding shown in the two kinds of societies. Perhaps the best explanation, Bailey said, may be that non-foraging societies are more likely to have heritable resources, such as wealth or livestock, so a tight-knit family group might be more likely to defend each other and their shared resources. By contrast, in a foraging society, it might make more sense to be part of a much larger, interconnected group, since there are few or no resources to be inherited.
It's tempting to think that people in agricultural societies might intermarry purely out of convenience, since they're less likely to encounter new people as often as hunter-gatherers might. But that's not the case, Bailey said. Agricultural societies tend to be much larger than hunter-gatherer ones, so if anything, the reverse might be expected.
[...] Still, it's hard to untangle whether there's a causal link between inbreeding and producing more children, Bailey said. Also, because marriages between two closely related individuals are rare in hunter-gatherer societies, the study was based on a small sample, which could have skewed the results.
Clearly, there needs to be more research. There are several things they're not considering.

For one, notice how in societies that regularly practice consanguineous marriage, such marriages are actually more reproductively successful. They didn't study disease rates, but rates for cousins usually aren't high. However, in societies where cousins frequently marry, they'd share a higher percentage of genes than otherwise. This study shows that we actually see an improvement in the number of surviving children.

One very significant possibility is that, as a result of their marriage practices, these societies have re-engineered their genes, and over time eliminated some of their damaging genes by exposing them through endogamy. The larger a population is, the less dangerous "inbreeding" is in the long run for the whole population. Some pockets have good genes while others have bad, and the higher rate of endogamy gives a selective advantage to those families with few harmful recessive genes. Is this actually the case, that, on average, ethnicities with a long history of consanguineous marriage have fewer harmful genes? Is "inbreeding" less dangerous in the long run for settled societies because they have larger populations?

There's another thing that doesn't get considered by academics: if, for whatever reason, there's a reproductive advantage to endogamy, then those who willingly engage in endogamy, and are more willing to have sex with their spouse, would be favored by natural selection. That means that, in settled societies, we should expect to see a higher percentage of people who feel sexual attraction for family. No-one knows what genes are responsible for the Westermarck Effect, so no-one can do any studies on population genetics to see which mutations are prevalent in which regions. The stereotype is that there's something "incestuous" about Europeans. Is it even true?

And after all, even among wild animals, it's been observed that rates of fertilization are higher when mates are closely related, because of the higher genetic compatibility between the mother and her zygote/fetus. Is this contributing to the findings?

If their hypothesis is true, then the primary difference in selective success for consanguineous couples is determined by social and economic forces. Exogamy is more socially and economically advantageous in small-scale, hunter-gatherer societies, while endogamy is more advantageous in large-scale, agrarian societies. Is this evidence for the mostly abandoned view, that socioeconomic forces are a prime driver for the development of "incest" taboos?

No-one knows.

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