We found evidence that neurogenesis and myelination—as well as genes expressed in the synapse, and those involved in the regulation of the nervous system—may explain some of the biological differences in intelligence.

re: nurture, hippocampal plasticity and hippocampal neurogenesis also appear to be affected by dancing and omega-3,6 (which are transformed into endocannabinoids by the body): https://news.ycombinator.com/item?id=15109698