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There are two main approaches to understanding the evolution of intelligence.
A) Study the differences in intelligence between genetic groups.
B) Study the differences in intelligence within a genetic group.
Approach A is currently not being funded, as far as I know, but please let me know if there are studies I should be commenting on. Approach B is where current research is concentrated. Usually, the hunt for the genes for intelligence has proceeded by comparing clever people with average people. This has yielded good results, though not as good as originally expected, when it was anticipated that a smallish number of “candidate” genes would be identified. In fact, the outcome has been more prosaic: “there are probably very many genes involved in building an intelligent brain, each playing a very small part, and probably each doing some other things as well”. Complicated. Given that apparent disappointment, it seemed a good idea to look at the genetics of extremely bright people, those with an IQ of 170+. In such a very bright sample the genes for intelligence should be easier to find. No, I have not contributed a sample of my saliva. I was not even asked to be a control, thus robbing me of the boast “I was a control subject in the study of genius”.
A genome-wide association study for extremely high intelligence. D Zabaneh, E Krapohl, HA Gaspar, C Curtis, SHLee, H Patel, S Newhouse, HMWu, MA Simpson, M Putallaz, D Lubinski, R Plomin and G Breen
Many of the authors will be known to you, particularly David Lubinski and Robert Plomin. The authors say:
We used a case–control genome-wide association (GWA) design with cases consisting of 1238 individuals from the top 0.0003 (~170 mean IQ) of the population distribution of intelligence and 8172 unselected population-based controls. The single-nucleotide polymorphism heritability for the extreme IQ trait was 0.33 (0.02), which is the highest so far for a cognitive phenotype, and signiﬁcant genome-wide genetic correlations of 0.78 were observed with educational attainment and 0.86 with population IQ. Three variants in locus ADAM12 achieved genome-wide signiﬁcance, although they did not replicate with published GWA analyses of normal-range IQ or educational attainment. A genome-wide polygenic score constructed from the GWA results accounted for 1.6% of the variance of intelligence in the normal range in an unselected sample of 3414 individuals, which is comparable to the variance explained by GWA studies of intelligence with substantially larger sample sizes. The gene family plexins, members of which are mutated in several monogenic neurodevelopmental disorders, was signiﬁcantly enriched for associations with high IQ. This study shows the utility of extreme trait selection for genetic study of intelligence and suggests that extremely high intelligence is continuous genetically with normal-range intelligence in the population.
The concept of being in the top 0.0003 of intellect is not an easy number to handle, not at my level of intelligence anyway. Does it help to say the top 0.03% ? Gigerenzer would suggest not. Decimals and percentages make strange bedfellows. Simplest to say the top 3 people in 10,000. Rare minds.
The authors continue:
One of the best-established ﬁndings in cognitive science is that individual differences in performance on diverse cognitive tasks correlate about 0.30 and that a general factor explains about 40% of the total variance. This general cognitive ability factor, usually called general intelligence (‘g’), is one of the best predictors of important life outcomes including education, occupation, and mental and physical health. General intelligence is also one of the most heritable behavioural traits, with heritability increasing from 40% in childhood to 80% in later adulthood.
The author set out their procedures, showing their quality controls and the way they establish significance which they have set at p<5×10 -8 so as to avoid the false positives which arise from multiple comparisons. This is another number which is difficult to handle, but the usual basic test for significance in social science is a mere p<5×10 -2. There are entire chapters which have been written about significance testing, and I am grateful to Prof Gerd Gigerenzer for having sent me his paper on this issue years ago.
The results show that the significant findings can be traced back to ADAM12 on chromosome 10, responsible for membrane-anchored proteins which have been implicated in a variety of biological processes involving cell–cell and cell–matrix interactions, including fertilization, muscle development and neurogenesis. These few significant findings can be seen in the top figure, poking their heads above common or garden genetic codes, or like helium filled balloons rising upwards to eminence.
Genetic correlations with intelligence were strong, and largest when they included a scholastic component
The key finding is that, from a genetic point of view, high intelligence is an extreme of normal intelligence. The estimated heritability of high intelligence is .33 the strongest ever demonstrated. In the pathway analysis, the gene family plexins, members of which are mutated in several monogenic neurodevelopmental disorders, was signiﬁcantly enriched for associations with high IQ (P-value = 6.43 × 10− 5). The plexin-semaphorin pathway has been linked to axon guidance, mental disability and neural connectivity, axon regeneration in the central nervous system, bone disorders, cancer and inﬂammatory diseases.
It is now standard in these genetics papers to move from sample of discovery to sample of testing. If only all other branches of psychology followed this “buy one, get one free” standard. The downside is that the variance accounted for is usually very small, as predictions which work in the original sample melt away when confronting the reality of a new sample.
A polygenic score created from the TIP GWA results accounted for 1.6% of the variance in individual differences in intelligence in TEDS at age 16 years for our 4-test composite of intelligence and 2.4% for our 16-test measure of g. A polygenic score created from the TIP GWA results accounted for 1.6% of the variance in individual differences in intelligence in TEDS at age 16years for our 4-test composite of intelligence and 2.4% for our 16-test measure of g.
This is a harsh test, because it is using only a very few signals to try to predict the whole range of normal intelligence. Of course, it is still disappointing that only a few snippets of genetic code emerge from the tough significance tests. The idea was that a hyper-bright sample would provide the rich vein of gold that had been sparse in other sedimentary rocks. One has the feeling that the secret of high ability remains, mostly, a secret. I hope that the authors will keep pressing on. Perhaps many of the marginally significant SNPs could be organized into an exploratory factor, and investigated further. Perhaps some new theories need to be advanced, such as may be derived from considering the building blocks of nervous systems in simpler organisms. Every code can be broken, but usually a crib is helpful. Armies send messages about ammunition, supplies and reinforcements. What do genes send messages about? (Yes, proteins, but proteins for what?)
The authors conclude:
In summary, we have shown that extremely high intelligence is a polygenic trait and its high heritability indicates that GWA analysis captures a large portion of the genetic variance. The novel aspect of the present study is that it represents a complementary strategy to the ‘brute force’ approach of increasing sample sizes of GWA studies of IQ variation in the normal range (and is an example for quantitative trait genetics in general). It demonstrates the utility of a ‘positive genetics’ strategy of focusing on the extremely high end of the distribution of IQ. Larger scale studies focusing on either high IQ or IQ in the normal range are likely to be successful in the identiﬁcation of many signiﬁcant loci and biological pathways.