Cousin marriage can reduce I.Q. a lot

By Razib Khan | July 20, 2012 12:33 am

In light of the previous post I was curious about the literature on inbreeding depression of IQ. A literature search led me to conclude two things:

– This is not a sexy field. A lot of the results are old.

– The range in depression for first cousin marriages seems to be on the order of 2.5 to 10 IQ points. In other words ~0.15 to ~0.65 standard deviation units of decline in intelligence.

The most extreme case was this paper from 1993, Inbreeding depression and intelligence quotient among north Indian children. The authors compared the children of first cousin marriages, and non-bred in individuals, from a sample of Muslims in Uttar Pradesh of comparable socioeconomic status (though the authors note that inbreeding has a positive correlation with socioeconomic status in this community). A table with results speaks for itself:


Comments (13)

  1. Jason Malloy

    If you’re looking for a more recent one, this French-language paper from 2009 is hard to find through Google Scholar. I’ll send you a large Indian one from 2004 too.

  2. As I mentioned in your other blog: how do you explain then that a markedly inbred population like Ashkenazi Jews appear to score slightly higher than average in IQ tests?

  3. Charles Nydorf

    The degree of homozygosity among Ashkenazi Jews is not especially high. Its no higher than that of Non-Jewish European-origin North Americans.

  4. zyprexa

    The two groups of parents (inbreeders v. non-inbreeders) were of comparable SES, but it is not clear that they were of comparable intelligence. That is, maybe the cousin-marriers had lower IQs than those who married genetically further away. If intelligence signals fitness, then lower IQ folks in the village would be less likely to attract desirable (i.e., higher IQ) mates from the next village over. That would leave them to mate with whom they could — their not-so-smart cousin who also couldn’t attract a non-related mate. So that would be assortative mating (two low IQ parents producing low IQ children) and not inbreeding depression.

  5. That’s weird because they clearly tend to form their own distinct cluster much like other highly inbred samples (Henn’s Tunisians for example, who seem to be a highly inbred oasis population after all) and also when compared with other West Asians and Jews (it also happens with Moroccan Jews but not with Sephardites, see here).

  6. dave chamberlin

    Years ago Cochran mentioned he had a theory as to why technological innovation in the Muslim world seemed to just peter out around 900 AD. He didn’t say what it was so I was left guessing. My first guess was the negative effect of first cousin marriage, far more common in the muslim world than elsewhere, if my memory serves me right from reading earlier GNXP threads on the subject of first cousin marriage. Hopefully he’ll follow up on these thoughts over at his blog West Hunter.

  7. Stephen

    How often do these studies distinguish between a first-cousin marriage in a “fully outbred” population, and one where cousin marriage is common and the mates are already both rather inbred AND somewhat related? Here’s a link to a prominent treatment where I believe they assumed the former:

  8. JL

    There’s an as-yet-unpublished study of the correlation between runs of homozygosity and IQ. I quoted the preliminary abstract here:

  9. That’s weird because they clearly tend to form their own distinct cluster much like other highly inbred samples

    if you take an admixed population, and turn it into a ‘sealed’ deme for ~1,500 years, then you get a very specific set of allelic differences vis-a-vis other populations.

  10. #2, in any case, inbreeding depression/outbreeding depression/hybrid vigor only deviates the expected value. that was my point about stratification. if you had an inbred, but tall, population, then more inbred individuals within the population may be shorter, but in the aggregate pool inbreeding and height would be positively correlated.

  11. Maybe, but what is the difference between “a sealed deme” and “an inbred population”? Of course I imagine that medieval Jews did not marry their cousins too often (although many peoples did even in Modern age, actually Darwin himself was that kind of several-generations-of-cousin-marriages inbred; of course that he was exceptionally gifted – randomness I presume – but he was also impelled to develop his theory for that reason).

    And what about a strong founder effect (which seems likely in the Ashkenazi case) followed by the “sealed deme” pattern (not so strictly sealed but anyhow), how isn’t that “inbreeding”? Something is amiss.

    Maybe it’s just a matter of degree and, as you say, there’s also an outbreeding depression, even if it’s not so obvious.

  12. but what is the difference between “a sealed deme” and “an inbred population”?

    ‘inbred’ is a human term that maps onto specific pop gen dynamics. e.g., the close relationship between inbreeding & genetic drift. for the purposes of this paper the dominant issue is that rare deleterious alleles are being brought back together, and reducing fitness. by this measure ashkenazi jews are definitely not inbred, except for perhaps some hasidic sects. there is debate about the jewish bottleneck. some researchers say there was one, others say not. jewish IBD seems likely a function of prolonged limited gene flow with other populations. over the generations all jews have ended up sharing the same ancestors over and over because of the limitations of their genealogical network since ~500 AD.

  13. Grey

    If industrialization and urbanization can break down inbreeding then the creation of an urbanized middle class from the brightest among inbred villagers could – if the same schools they and their children went to were used to test for IQ results over time – show increasing IQ over time – at least in those schools. If so then the effects of this process would vary depending on whether it started in 1850, 1900, 1950 etc in that nation.

    Another thought is regional or national caricatures of relative average stupidity – especially of people from the most rural areas – may have been true at the time those caricatures were created and became untrue later.


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About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at


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