Wednesday 8 July 2015

Not tonight, cousin

Although psychology is still a cottage industry, Edinburgh seems to be leading the scholarly equivalent of a global corporate behemoth, churning out blockbuster productions with author lists longer than the average paper, and sample sizes greater than many principalities.

With the enigmatic title: Directional dominance on stature and cognition in diverse human populations the paper just sits there, like a new jet engine for a plane which has yet to be built. Let me walk you round this gleaming contraption in awe, and tell you what I can discern of its surface characteristics.

Directional dominance means that all the genes show dominance in the same direction. If you marry close relatives, such as cousins, you increase the Bad Bingo chance that a bad mutation in the father will line up exactly with a bad mutation in the mother to give the poor child a double dose of trouble. (Of course, the stultifying conventions of academic publication do not allow anyone to say anything as plain as that).



The authors, whose name is Legion, say:  Genomic data now allow us to investigate the effects of homozygosity on traits of public health importance by observing contiguous homozygous segments (runs of homozygosity), which are inferred to be homozygous along their complete length. Given the low levels of genome-wide homozygosity prevalent in most human populations, information is
required on very large numbers of people to provide sufficient power. Here we use runs of homozygosity to study 16 health-related quantitative traits in 354,224 individuals from 102 cohorts, and find statistically significant associations between summed runs of homozygosity and four complex traits: height, forced expiratory
lung volume in one second, general cognitive ability and educational attainment. In each case, increased homozygosity was associated with decreased trait value, equivalent to the offspring of first cousins being 1.2 cm shorter and having 10 months’ less education.

In plain language, runs of homozygosity caused by inbreeding have significant deleterious effects on body and mind.

Similar effect sizes were found across four continental groups and populations with different degrees of genome-wide homozygosity, providing evidence that homozygosity, rather than confounding, directly contributes to phenotypic variance.

The effect is solid across races and different degrees of inbreeding.

Contrary to earlier reports in substantially smaller samples, no evidence was
seen of an influence of genome-wide homozygosity on blood pressure and low density lipoprotein cholesterol, or ten other cardio- metabolic traits.

Cardio-vascular traits are not affected by inbreeding.

Since directional dominance is predicted for traits under directional evolutionary selection, this study provides evidence that increased stature and cognitive function have been positively selected in human evolution, whereas many important
risk factors for late-onset complex diseases may not have been.

For many generations taller and brighter persons have been chosen as better mates.

(Perhaps, late in life, I could get a job as a translator of scientific papers).

Here are some more findings: Isolated populations have a higher burden of homozygosity, Africans least homozygosity. The Amish are among the most inbred, with other small communities in the US and elsewhere close to those levels.




Here are their findings on intelligence:  Our genomic confirmation of directional dominance for g and discovery of genome-wide homozygosity effects on educational attainment in a wide range of human populations adds to our knowledge of the genetic underpinnings of cognitive differences, which are currently thought to be largely due to additive genetic effects. Our findings go beyond earlier pedigree-based analyses of recent consanguinity to demonstrate that the observed effect of genome-wide homozygosity is not a result of confounding and influences demographically diverse populations across the globe. The estimated effect size is consistent with pedigree data (a 0.01 increase in F decreases g by 0.046 s.d. in our analysis and 0.029–0.048 s.d. in pedigree-based studies)20. It is germane to note that one extreme of cognitive function, early onset cognitive impairment, is strongly influenced by deleterious recessive loci, so we can speculate that an accumulation of recessive variants of weaker effect may influence normal variation in cognitive function.

Here is their last word:

We have demonstrated the existence of directional dominance on four complex traits (stature, lung function, cognitive ability and educational attainment), while showing any effect on another 12 health-related traits is at least almost an order of magnitude smaller, non-linear or non-existent. This directional dominance implies that size and cognition (like schizophrenia protective alleles) have been positively selected in human history – or at least that some variants increasing these traits contribute to fitness.

Breeding with closely related persons reduces the height, intelligence and lung capacity of your children. Other stuff, like cardio-vascular health, is probably unaffected. 

What fills me with admiration is the way that these very large data sets are being assembled, enabling the signal to stand out from small sample noise. Looking at the genetics of intelligence has become commonplace, and I hope that comparisons across continents will also become habitual, thus putting genetic hypotheses to the test.

There is more here for the sharp-eyed reader, but the general picture is clear: Keep your reproductive distance from cousins and other related persons, if you value intelligence, that is.


  1. "traits of public health importance" amused me, since they report on a job lot of traits the importance of which is contentious. Still, those who flog statins, anti-hypertensives, and metformin, will be happy. Put otherwise, that figure seems to me to suggest that a dozen much fussed about measurements may scarcely matter at all.

  2. "(Perhaps, late in life, I could get a job as a translator of scientific papers)."

    Yes, very handy.

  3. Endogamy have a greater evolutionary approach = found new human varieties.

    I think generally poor (and less smart) and rural people who are more prone to engage in endogamic patterns.

  4. Do any of these studies establish the degree (first, second, and so on) of cousins where these risks are elevated? I read somewhere maternal cousins are more of a health risk. Is that true?

  5. Inbreeding measured by the f coefficient from Sewall Wright, and f values given in the paper and mentioned in my summary. This is for a hypothetical gene. Don't know about the specific issue of maternal cousins.

  6. Is there any report on IQ effects or in convertible units?

  7. Off topic: this morning's Telegraph carries some comments about psychological testing by John Timpson of the shoe-repair chain. I thought they might entertain you.

    1. Many thanks. A good read. He has sussed out the limitations of assessment factory reports.