A new study by Sanja Franic and his colleagues, published in the prestigious journal Intelligence found no link between genes and intelligence. The authors write:

Using an existing pool of known intellectual disability genes, we constructed a set of 168 candidate genes for normal-range intelligence, and tested their association with intelligence in 191 individuals (aged 5–18) sampled from the high and low ends of the IQ distribution. In particular, we 1) employed exon sequencing to examine the possible effects of rare genetic variants in the 168 genes, and 2) used polygenic prediction to examine the overall effect of common genetic variants in the candidate gene set in a larger sample (N = 2125, mean age 20.4, SD = 14.1). No significant association between the candidate gene set and intelligence was detected.

Of course I’m not surprised this study found nothing, because genes that cause organic retardation (which are presumably what they looked at) should have little to do with normal biological variation in intelligence, because organic retardation, by definition, should have an exceptional cause.

Instead they should have been looking at the genetic causes of familial retardation, because familial retardates are just the extreme low end of the bell curve of the biologically normal population that has a mean of 100, while organic retardates are a different population with their own bell curve, with a mean of about 50. Just as some biologically normal people have IQs below 50, some organics have IQs above 100, but the two groups are qualitatively different, even when they are quantitatively the same (i.e. matched on IQ).

Of course we don’t know the genetic causes of familial retardation which is precisely the problem. Scientists have only discovered the causes of organic retardation, not biologically normal variation in intelligence.

An ideal study, would be to compare the DNA of biologically normal people from around the world who are say 50 IQ points smarter than others of their country and ethnic group, with those who score 50 IQ points lower than others of their country and ethnic group. But it would be essential that those scoring 50 points lower are familial retardates, not organic retardates. As for those scoring 50 points higher, it’s generally assumed that giftedness is always familial, but if organic giftedness exists, they should be excluded from the study too.

To understand the difference between organic retardates and familial retardates, consider the difference between organic giants and familial giants. Familial giants are biologically normal people who are over seven feet tall. Because they are biologically normal, they have all the traits associated with height. They tend to be men. They tend to be black or caucasoid. They tend to be good at basketball. They tend to have tall siblings. They tend to have lots of girlfriends and radiate good health.

By contrast organic giants tend have something wrong with them like a pituitary disorder. They are the same height, but they tend to look odd, they are bad at sports, they don’t get many dates, they don’t have tall siblings. Even people from demographic groups we don’t consider especially tall (i.e. Chinese women) can be organic giants.

Analogously, organic low IQ people tend to have something wrong with them like an extra chromosome, and they tend to look very different, and do not have the background normally associated with low IQ; they are virtually no more likely to come from low IQ families or low IQ demographics than anyone else.

As a result, genes or genetic variants that causes organic retardation can be very different from genes or genetic variants that cause normal intelligence variation. A good example might be myopia. It is believed that high IQ and myopia both have a common genetic cause. But as scholar Arthur Jensen has noted, people with Down’s Syndrome show the same rates of myopia as everyone else despite averaging IQs around 50. This is because Down’s Syndrome is organic and thus overrides many of the normal genetic causes of IQ. But this is precisely why focusing on organic retardates can be so misleading when searching for the causes of biologically normal IQ variation.

It used to be thought that high IQ people had myopia because they did a lot of reading and other “near work”, but the fact that Down’s Syndrome people have the same rate of myopia as the general population helped debunk such environmental explanations.