The genes that support cognition are usually the same genes implicated in autism and adhd. Most variants imparting increased risk for those conditions are extre

The genes that support cognition are usually the same genes implicated in autism and adhd. Most variants imparting increased risk for those conditions are extremely common,(typical odds ratio, <<1.1). Some rare variants impart greater risk but those variants are uncommon, in part because of natural selection. If you do not have these rare variants, it typically takes a large number of common mutations before you get the condition. Thus natural selection effects against those rare variants are weak, and this ignores that there might also be beneficial effects of those common variants, either at the group level by increasing diversity of thought, or when appearing without other rare variants. The take home is that it is much more complicated than single gene disorders where selection effects can have their greatest impact. Common variants impact on fitness are heavily conditioned on the overall genetic mileau.