In 2011, a large group of researchers published the results of a genome-wide analysis of 549,692 single nucleotide polymorphisms (SNPs) involving 3511 unrelated adults. (An SNP represents a difference in a single DNA building block, called a nucleotide.) They reported:
Our results unequivocally confirm that a substantial proportion of individual differences in human intelligence is due to genetic variation, and are consistent with many genes of small effects underlying the additive genetic influences on intelligence.
This research estimated the heritability of IQ to be about 0.5, confirming the results of the studies involving twins. Its conclusion that general intelligence is polygenic, i.e., it derives from a combination of many genes supports the concept of intelligence as a multi-faceted characteristic.
This means that there are not a “handful” of genes responsible for intelligence, but rather thousands of SNPs that in combination contribute to brain function and development. Since each of these SNPs contributes a minuscule amount to cognitive function, it is difficult to isolate the contribution of each SNP by linking each to performance on intelligence tests.
Control experiments matching a specific SNP to cognitive function requires huge samples of people to whom tests are administered followed by complex statistical techniques to draw reliable inferences. However, this analysis is further complicated by the fact that gene expression can be significantly affected by environmental influences, meaning that two people with similar genetic constitution, may score differently on intelligence tests on account of epigenetic factors.
One method of isolating the effects of particular genes is a process known as CRISPR that searches for a particular sequence of DNA and then performs a kind of “gene splicing” that removes one gene and replaces it with another. By comparing the “before” and “after” effects of the splicing, it is possible to analyze the function of a particular gene. For example, CCR5, a gene that contributes to immune function, when edited in animal models, reveals changes in brain plasticity.
The process of connecting genes with their function is further complicated by the fact that some genes that affect intelligence may only display effects during the developmental phase of intelligence and would have no effect if gene splicing were applied to an adult. It seems that determining the genes that contribute to specific facets of intelligence is even more difficult than determining intelligence itself.
