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Konkvistador comments on Poly marriage? - Less Wrong Discussion

-9 Post author: h-H 06 June 2012 07:57PM

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Comment author: gwern 07 June 2012 06:16:41PM 7 points [-]

Incest laws in general makes no sense.

I believe I've previously pointed out inbreeding depression to you; from Jensen 1969:

In Japan approximately 5% of all marriages are between cousins. Schull and Neel studied the offspring of marriages of first cousins, first cousins once removed, and second cousins. The parents were statistically matched with a control group of unrelated parents for age and socioeconomic factors. Children from the cousin marriages and the control children from unrelated parents (total N=2111) were given the Japanese version of the Wechsler Intelligence Scale for Children (WISC). The degree of consanguinity represented by the cousin marriages in this study had the effect of depressing WISC IQs by an average of 7.4%, making the mean of the inbred group nearly 8 IQ points lower than the mean of the control group. Assuming normal distributions of IQ, the effect is shown in Figure 9, and illustrates the point that the most drastic consequences of group mean differences are to be seen in the tails of the distributions. In the same study, a similar depressing effect was found for other polygenic characteristics such as several anthropometric and dental variables.

The mating of relatives closer than cousins can produce a markedly greater reduction in offspring's IQs. Lindzey (1967) has reported that almost half of a group of children born to so-called nuclear incest matings (brother-sister or father-daughter) could not be placed for adoption because of mental retardation and other severe defects which had a relatively low incidence among the offspring of unrelated parents who were matched with the incestuous parents in intelligence, socioeconomic status, age, weight, and stature.

No doubt one could dig up more recent results with a suitable search.

Comment author: [deleted] 07 June 2012 06:23:10PM *  6 points [-]

Should we use eugenic reasons forbid the marriage of women over 40 planing on having babies as well? The defect rate in children is the same just so you know. In the Western world most people are pretty out-bred, one generation of inbreeding does very little damage and very few Westerners will choose to marry their cousins generation after generation.

Comment author: gwern 07 June 2012 06:29:25PM 12 points [-]

Should we use eugenic reasons forbid the marriage of women over 40 planing on having babies as well?

Yes. We should also encourage the use of sperm donors as resulting in 1/5 the birth defect rate. (On a large scale, this advantage would erode, but that is easily dealt with by encouraging men to donate more and treating them less like crap; we can think of it as 'sperm banking', akin to existing practice of 'egg banking'.)

In the Western world most people are pretty out-bred, one generation of inbreeding does very little damage

Do you know this, or are you guessing? If I were to go looking for any studies in out-bred populations, at how many points of average IQ damage would you concede the point?

Comment author: [deleted] 07 June 2012 06:30:49PM *  5 points [-]

Yes. We should also encourage the use of sperm donors as resulting in 1/5 the birth defect rate.

Ok you win eugenics is a good idea. But after designer babies are the norm, this rationale becomes obsolete.

Comment author: [deleted] 07 June 2012 07:33:49PM *  1 point [-]

In the Western world most people are pretty out-bred

By “the Western world” you mean the Americas or some subset thereof? Because I doubt there's that much genetic diversity in, say, Iceland (and in Italy there are some small towns where one or two family names comprise the majority of the population).

Comment author: [deleted] 08 June 2012 05:29:36AM *  4 points [-]

The Western world. You need to keep in mind that inbreeding is the human norm. Europe, at least any part that was under Catholic influence for a few centuries is a massive outlier in its low rates of consanguineous marriage.

Comment author: [deleted] 08 June 2012 07:37:51AM 2 points [-]

Europe, at least any part that was under Catholic influence for a few centuries is a massive outlier in its low rates of consanguineous marriage.

They might not marry close relatives (say, those with common ancestors from 200 years ago), but when it's customary to only marry people from your own town with a population of 1000, this means that lots of your ancestors from 500 years ago are your spouse's ancestors too, so you still share lots of genes.

Comment author: [deleted] 08 June 2012 07:41:01AM *  3 points [-]

Urbanization in the 19th and 20th century mean this is very seldom still the case. How many people live in such small towns? And those who more often find spouses via work or in the course of their education, which seldom takes place in such small towns.

In any case the defect rates for such towns are only very seldom a problem. A better point in your favour might be the relative close relatedness of say the Aksenazi Jews.

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