Robert Plomin on the genetics of various mental traits (source)
A Chinese research team is looking for genes that explain why IQ is higher in some people and lower in others:
Studies show that at least half of the variation in intelligence quotient, or IQ, is inherited. But while scientists have identified some genes that can significantly lower IQ—in people afflicted with mental retardation, for example—truly important genes that affect normal IQ variation have yet to be pinned down.
The Hong Kong researchers hope to crack the problem by comparing the genomes of super-high-IQ individuals with the genomes of people drawn from the general population. By studying the variation in the two groups, they hope to isolate some of the hereditary factors behind IQ. (Naik, 2013)
The head of the team, Zhao Bowen, believes this question has not been resolved because it is too controversial. “People have chosen to ignore the genetics of intelligence for a long time," said Mr. Zhao, who hopes to publish his team's initial findings this summer. "People believe it's a controversial topic, especially in the West. That's not the case in China" (Naik, 2013).
Perhaps. But there is another reason: the apparently large number of genes involved and the relatively small effects of each one. This was the conclusion of Robert Plomin, a behavioral geneticist at the Institute of Psychiatry in London:
Failing to find genes for intelligence has, in itself, been very instructive for Plomin. Twin studies continue to persuade him that the genes exist. “There is ultimately DNA variation responsible for it,” he says. But each of the variations detected so far only makes a tiny contribution to differences in intelligence. “I think nobody thought that the biggest effects would account for less than 1 percent,” Plomin points out.
That means that there must be hundreds—perhaps thousands—of genes that together produce the full range of gene-based variation in intelligence. (Zimmer, 2008)
This should be no surprise. Natural selection doesn’t act on genes, at least not directly. It acts on phenotypes—the flesh-and-blood outcomes of genes. Selection for intelligence will thus affect any gene that has some kind of intelligence-boosting effect. This point has been made by Linda Gottfredson, a psychology professor at the University of Delaware:
[...] within-group ('individual") differences in intelligence will involve 1000s of genes of small effect, so we can expect that for between-group differences too. Many of the genes will not be specific to intelligence per se but influence broad physiological processes that affect brain structure and function. This would include cardiovascular fitness and much more. (Go exercise, guys!)
I read that perhaps half our genes are expressed in the brain. If half of our segregating genes are too (the 0.1% on which humans are estimated to differ), that's still 1.5 million base pairs or "SNPs" (of 3 billion total).
This indicates the challenge, even if we ignore other important genomic differences (e.g., number of times a given segment of the chromosome is repeated, like a stutter).
[...] This is not to say, of course, that we can't pin down heritabilities for various mean group differences (we could right now if researchers were willing) or that we won't be able to identify numbers or classes of genes on which groups differ most. But it's looking unlikely that we'll be able to pinpoint a list of specific genes that explain much of the normal variation in g, either within or between groups. (Gottfredson, 2013)
Interestingly, Robert Plomin is mentioned as one of the people involved in the Chinese project. Has this research been offshored to a country where the intellectual climate is less restrictive? In addition, since Plomin is aware of the limitations of this kind of study, he might know something that the rest of us don’t. Perhaps among the many genes with small effects there are a few with big effects …
Gottfredon, L. (2013). H-bd discussion list, February 22, 2013
Naik, G. (2013). A genetic code for genius, The Wall Street Journal, February 15http://online.wsj.com/article/SB10001424127887324162304578303992108696034.html?mod=WSJ_hp_mostpop_read
Zimmer, C. (2008). The search for intelligence. Scientific American, October, pp. 68-75.