What are the causes of differences in intelligence? Twin and adoption studies show that intelligence is about 50% heritable. Counter-intuitively, heritability estimates increase to 80% as you age and put many years between yourself and your family’s influence on you. It looks like genes take time to express themselves, or set in motion processes which take time to develop.
If you measure a diverse range of skills you can show that g is highly heritable, but there is less genetic influence that is specific to each domain. Heritabilities are probably higher in richer people, and lower in deprived groups (suggesting the environment has most effect when it is very bad).
There is no “gene for intelligence” with the exception of the APOE gene, where individuals with one or two e4 alleles tend to have lower ability in old age, and declining cognition across their lifetimes. (Reassuring that my condition has a name).
Genome wide assessment studies haven’t come up with very much, yet. There is some molecular genetic evidence that some variance in intelligence is detected by single nucleotide polymorphisms. Applying genetic complex trait analysis, between a quarter and a half of intelligence variance can be accounted for by variants in linkage disequilibrium with common SNPs. This analysis cannot identify the precise causal genes. It suggests that intelligence is highly polygenic, with large numbers of variants of small effect sizes.
The genetic correlation between intelligence measured in childhood and old age in the same individual is high: to a substantial extent the same genes cause higher intelligence in both childhood and old age. All these studies require very large sample sizes (125,000 is a good number) and allow major risks to be computed: in individuals who are at genetic risk for schizophrenia, but have not exhibited the disorder, cognitive ability in old age is lower than in those without schizophrenic genetic risk.
With regards to the much vaunted effects of the environment, twin studies suggest that the contribution of the shared environment (family for example) to intelligence is small to negligible by adulthood, and the remaining variance is due to individually created environments and error.
Brain correlates of intelligence differences
There is a general finding that there is a modest correlation of 0.30 between intelligence test scores and brain size, and a similarly sized correlation between intelligence and the general integrity of the brain’s white matter, as measured by diffusion tensor MRI. Sample sizes here are of the order of 500 persons. The association is largely accounted for by people’s differences in speed of processing. Cleverer brains seem to be more efficient. Consequently, they can think about complex problems faster and for longer, thus being more likely to find solutions.
You probably already knew that.