Pedigree collapse and inbreeding

Alex Shoumatoff:

Pedigree collapse is caused by cousins marrying cousins: both intentional mating between close cousins and random mating between distant ones who don't know that they are related. Close-cousin marriage has happened much more often than is generally supposed. In tribal societies, the exogamic restriction is usually applied not to all one's blood relatives but only to those in one's kinship group. In a patrilineal society, for instance, there is nothing to stop one from marrying a matrilineal cousin or uncle; in fact, such a match is often esteemed. The ideal is to marry out, but not too far out. In Japan, which has one of the world's highest consanguinity rates, arranged marriages between first cousins have been going on for centuries; and surveys made in the nineteen-six ties in southern India found that up to a third of the marriages among the Sudras of Andhra Pradesh were between first cousins and that the proportion of uncle-niece matings might have been as high as twelve per cent.

"If we could only get into God's memory, we would find that eighty per cent of the world's marriages have been with at least second cousins," the British social theorist Robin Fox told me recently. "In a population of between three and five hundred people, after six generations or so there are only third cousins or closer to marry. During most of human history, people have lived in small, isolated communities of about that size, and have in fact probably been closer to the genetic equivalent of first cousins, because of their multiple consanguinity. In nineteenth-century rural England, for instance, the radius of the average isolate, or pool of potential spouses, was about five miles, which was the distance a man could comfortably walk twice on his day off, when he went courting - his roaming area by daylight. Parish registers bear this out. Then the bicycle extended the radius to twenty-five miles. This was a big shakeup."

"Pedigree collapse" due to inbreeding

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