We identify several common genetic variants associated with cognitive performance using a two-stage approach: we conduct a genome-wide association study of educational attainment to generate a set of candidates, and then we estimate the association of these variants with cognitive performance. In older Americans, we find that these variants are jointly associated with cognitive health. Bioinformatics analyses implicate a set of genes that is associated with a particular neurotransmitter pathway involved in synaptic plasticity, the main cellular mechanism for learning and memory. In addition to the substantive contribution, this work also serves to show a proxy-phenotype approach to discovering common genetic variants that is likely to be useful for many phenotypes of interest to social scientists (such as personality traits).
Ewen Callaway has a write up in Nature. The issue here is that it’s been evident for the past 10 years or so intelligence variation is not due to alleles segregating at high frequencies with at least modest effects, so they’re hard to pick up in association studies (contrast with pigmentation, which is mostly controlled by a number of loci on the order of 10). Some, such as Kevin Mitchell, don’t think that common variants are the way to go, period. Common as in variants which are found across the population, even if their effect on the trait is very small. This group disagrees. One of the authors, Peter Visscher, has written up his own view of this line of research, Intelligence inheritance – three genes that add to your IQ score:
This study of normal variation in cognitive performance confirms that there is no gene with a large effect on this trait. There is no “gene for intelligence” – instead, cognitive performance is likely to be influenced by thousands of genes, each having a small effect.
While the individual effect of the genetic variants are extremely small, their identification may lead to knowledge of the biological pathways involved in cognitive performance and cognitive ageing. This insight may eventually lead us into a better understanding of the mechanism involves in memory loss and dementia.
Finally, because individual gene effects are small, an implication of the study is that even larger studies, for example on millions of people, will lead to the discovery of many more gene variants.
In sum, because intelligence is at least moderately heritable, but the causal variants are so diffuse and numerous, the best bet for having a smart child is picking a spouse with a deviated phenotype. Look for smart people to marry….