IQ and Genetic Amplification

I was right (about this).

The increase in the heritability of intelligence may be the result of several processes. Genetic amplification has been previously suggested (DeFries et al., 1987), and this is what was observed in the present study….

….How the increase in genetic influences on intelligence is associated with developmental brain changes that occur around the period of transition from childhood to adolescence is still not fully understood. Measures for brain anatomy are under large genetic influences throughout early infancy (Gilmore et al., 2010; Smit et al., 2010), childhood and adolescence (Smit et al., 2010; Peper et al., 2009; Yoon et al., 2010; Brouwer et al., 2010; Schmitt et al., 2007; Wallace et al., 2006), and adulthood (Peper et al., 2007; Baare et al., 2001). While the genetic influences that explain individual differences in brain size seem to be generally stable at different ages, the human brain itself is a highly dynamic organ, and undergoes considerable devel- opmental changes during development from infancy up to adulthood (Giedd et al., 1999). The genetic influences found for variation in brain anatomy showed an overlap with genetic influences on intelligence in adults (Posthuma et al., 2002; Hulshoff Pol et al., 2006), and in childhood and adolescence (van Leeuwen et al., 2009; Betjemann et al., 2010; Wallace et al., 2010). The amount of brain changes in cortical thickness in adults is under genetic influences, and partly overlap with genetic influences on IQ (Brans et al., 2010). There are indications that the level of intelligence is associated with developmental trajectories of the human cortex during adolescence (Shaw et al., 2006). (Heritability of Verbal and Performance Intelligence in a Pediatric Longitudinal Sample)

I pointed out, a while back, that the overwhelming evidence of the high heritability of g and other traits has led more savvy environmentalists to reinvent themselves as GE correlationists. A GE correlational model of IQ (and other) differences is just an environmental model with a genetic veneer. So, as a result of the self-reinventions, you now frequently get “lost in correlations” bits like Flynn’s “the heritability of basketball.” Here’s an excerpt from Dickens’ article, Cognitive Ability, which makes the same point:

So how is it that large gains are possible in the face of high heritability estimates? The chief flaw in the argument that high heritability implies a limited role for environment is that it misunderstands what heritability is measuring. It ignores the possibility that genetic and environmental influences might be correlated. In particular, it ignores the possibility that genetic influences on ability are largely the work of environmental advantages that come about due to modest physiological advantages.
Consider a sports analogy.

Not infrequently, it was claimed that the increasing heritability of IQ with age supported a correlational model — heritability increases because as they age people are more and more free to seek out the cognitive niches which match their dispositions. The most plausible alternative model for this increase, which eerily often went and still goes unmentioned, is one involving genetic amplification. As for going unmentioned, take this passage from Haworth, Wright. Luciano, Martin, de Geus, van Beijsterveldt, Bartels, Posthuma, Boomsma, Davis, Kovas, Corley, DeFries, Hewitt, Olson, Rhea, Wadsworth, Iacono, McGue, Thompson, Hart, Petrill, Lubinski and Plomin (2009), which is stunning in this regards:

Why, despite life’s ‘slings and arrows of outrageous fortune’, do genetically driven differences increasingly account for differences in general cognitive ability during the school years? It is possible that heritability increases as more genes come into play as the brain undergoes its major transitions from infancy to childhood and again during adolescence. However, longitudinal genetic research indicates that genes largely contribute to continuity rather than change in g during the school years. We suggest that the developmental increase in the heritability of g lies with genotype–environment correlation: as children grow up, they increasingly select, modify and even create their own experiences in part on the basis of their genetic propensities. (The heritability of general cognitive ability increases linearly from childhood to young adulthood)

No mention of that other possible explanation for the 0.5 increase in heritability from infancy to adulthood? (Two of you did first develop the idea in the 80s.)

All of this should now be a moot point since Davies et al. (2011) has established the high narrow heritability (i.e., breeding value) of IQ. (If the high heritability or increasing heritability found in conventional twin studies was a function of (passive/evocative/active) GE correlations, as was argued, Davies et al. (2011) would have found a much lower narrow heritability in their study.) Though, the import of Davies et al. (2011) seems to have been lost on some. It is somewhat disturbing that the two dozen or so behavioral geneticists and psychometricians cited above were so off the mark with their (fairly recent) suggestion that “the developmental increase in the heritability of g lies with genotype–environment correlation.” Though, when reading the passage, one gets the feeling that they were trying to appease the environmentalist gods. Whatever the case, they were wrong and I was right.

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7 Responses to IQ and Genetic Amplification

  1. nikcrit says:

    Whoa alert!

    I came across this remark within the comments of a current post at Half Sigma; I’m sure you’ll be interested in checking this out and perhaps offering a corrective on your behalf:

    “The blog Occidental Ascent is run by a self described liberal who’s deeply into HBD.”

    Here’s the link to the page:

  2. LH says:

    So.. all interesting stuff, but how might an interested person get a broader knowledge of this whole subject area more directly, i.e. through three of four good books, rather than reading smaller, zig-zagging pieces across the HBD blogosphere?

    Read the Bell Curve and On Genetic Interests and one or two items but neither of these dive into the genetics / mechanics. Of course a biological textbook can outline the very basics but I have those and a basic understanding of all that. I’m more interested in the genetics allied with the population aspects. Any readers or blog owners like to give some direction?

    • Chuck says:


      I’m not sure what you’re looking for. If you’re looking for works that deal with the overlap between population and behavioral genetics, you’re not going to find many, since this is a little researched (and somewhat taboo) area. A while back, I did compile a list of books and articles on related subjects that may be of some help. Here is another compilation.

  3. LH says:

    Yep, that and a little wider Chuck, thank you for these lists. Will dig into them.

  4. Kiwiguy says:

    Have you have emailed Plomin & De Fries about this?

  5. LibHer says:

    I remembered your article when reading this recent interview and while it is of course aimed at a non-scientific audience, I thought I would pass it along regarding some comments of Plomin’s to the interviewer regarding British GCSEs and IQ (though not race so much).

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