Dr. Goldstein has also found that common genetic variants do not explain variation in IQ, at least not among different human populations:
He says he thinks that no significant genetic differences will be found between races because of his belief in the efficiency of natural selection. Just as selection turns out to have pruned away most disease-causing variants, it has also maximized human cognitive capacities because these are so critical to survival. “My best guess is that human intelligence was always a helpful thing in most places and times and we have all been under strong selection to be as bright as we can be,” he said.
This is more than just a guess, however. As part of a project on schizophrenia, Dr. Goldstein has done a genomewide association study on 2,000 volunteers of all races who were put through cognitive tests. “We have looked at the effect of common variation on cognition, and there is nothing,” Dr. Goldstein said, meaning that he can find no common genetic variants that affect intelligence. His view is that intelligence was developed early in human evolutionary history and was then standardized.
The finding itself is not surprising. The human brain is a complex organ with more than a trillion nerve cells. Clearly, a lot of genes are brain-related. If natural selection has caused one such gene to vary from one human population to the next, the same selection pressure has probably caused others to vary as well. Thus, in the event that human populations differ genetically in cognitive performance, the overall difference should reflect an accumulation of small differences at many gene sites—often too small to measure.
But what about g? Doesn’t g imply that one gene accounts for most genetic variation in intelligence? Perhaps. Alternately, g may correspond to a large number of brain genes that co-vary because they lie next to each other on the genome. In any case, the chances are not good that we will find g by trolling through the common variants we have discovered so far. The genome is a big place. Such a random search would be like looking for a needle in a haystack.