There have long been strong grounds for acknowledging a substantial genetic contribution to intelligence. At heart the argument is simple: there is a correlation between relatedness and intelligence, strongly discernible between identical twins, less so between fraternal twins and parents and children, though still significant. And thereby hangs the problem: there is a correlation, but correlations are slippery things, explicable in different ways. Families share genetics, but also experiences, so the contributions of each can be hard to disentangle. Separated twin studies and adoption studies can help attribute the source of variance, but not to the satisfaction of environmentalists.
In this football match it is Hereditarians versus Environmentalists. The hardline Hereditarians say that intelligence is 80% genetic in countries with good standards of living, and 50% genetic in poor countries. Hardline Environmentalists (Richard Nisbett) say that intelligence is 100% due to the environment. Those Environmentalists who are less hardline would say that there may be some genetic effects, but that they are small. Of course, variance estimates depend on the samples, the measures and the circumstances, but a general pattern has emerged, and the debate is about how to interpret that pattern.
Science depends on proofs, so 15 years ago a number of researchers led by Bob Plomin set out to find the genes for intelligence. They argued that they had got to be there, somewhere. Their strategy was to collect DNA from very bright people, and compare the pattern with the DNA extracted from average people. Nothing showed up. The problem is that there are no genes for intelligence, any more than you would expect to find genes for romantic poetry. The behaviours we regard as intelligent involve the whole brain. Roughly half the human genome is expressed in brain. In all likelihood there are very many genes which each have only a tiny effect and the end result is intelligent behaviour to varying degrees. Those tiny effect genes are scattered among many others, and thus have proved almost impossible to find.
Warning: what follows a gross simplification. The full paper can be obtained here: http://pss.sagepub.com/content/24/4/562
In those early days genetic analysis was a cumbersome, expensive and slow business. Cracking the first human genome took 10 years, lots of money, and many whirring, shaking, humming machines, where serried ranks of automated pipettes moved with hypnotic regularity through multiple samples. Now the analysis can be done by a chip the size of a postage stamp (the Affymetrix 6.0 DNA array) and you get lots of additional data. It can hunt for minute signals in a wall of genetic sound, and given bigger samples it should one day get an even stronger signal, and thus identify many of the genetic variants that influence cognitive abilities.
This new approach, genome-wide complex-trait analysis (GCTA), does not try to identify individual genes but instead estimates the total heritability captured by common DNA markers across an entire population sample. Using 3,154 pairs of 12-year-old twins they worked out estimates on the basis of 1.7 million DNA markers. The DNA markers tagged by the array accounted for .66 of the estimated heritability, reaffirming that cognitive abilities are heritable. There is still one third of the estimated heritability to be identified, but that may be achievable soon, once newer analytic chips can identify rare variants which are being missed at the moment.
This study is part of a pattern, with other teams showing there are many genes involved, each having small effects. Ian Deary’s group has achieved results of comparable magnitude, by related methods on a different population. No individual study will be sufficient but in Plomin’s view this technique might also mark “the beginning of the end of the nature-nurture controversy because it is much more difficult to dispute DNA-based evidence for genetic influence than it is to question the results of twin and adoption studies”.
So, the Hereditarians have taken a step forward. Their explanations about the genetics of intelligence can now be grounded in patterns of DNA. There is much more to be done, but the next few years should be interesting, probably even astounding.