Recent large sample within-family data does seem to establish causal effects of brain size on intelligence and educational attainment. The genetic correlation is ~0.4, so most of the genetic variance isn’t working through overall brain size.
Some kinds of features that could contribute to genetic variance in humans, but not scale for arbitrary differences across species:
Mutation load (the rate at which this is trimmed back, and thus the equilibrium load, depends on the strength of selection for cognitive abilities)
Motivation: attention to learning, play, imitation, and language comes at the expense of attention to other things
Pleiotropy with other selection combined with evolutionary limits (selection for lower aggression also causes white patches in fur via changes in neural crests, and retention of a variety of juvenile features), e.g. selection for disease resistance changing pathways so as to accidentally impair brain function (with the change surviving because of its benefits)
Alleles that provide resistance to disease (genetic variance is maintained in a Red Queen’s Race) that damages the brain would be a source of genetic variance, likewise variants affecting nutrition or other environmental influences
Recent large sample within-family data does seem to establish causal effects of brain size on intelligence and educational attainment. The genetic correlation is ~0.4, so most of the genetic variance isn’t working through overall brain size.
Some kinds of features that could contribute to genetic variance in humans, but not scale for arbitrary differences across species:
Mutation load (the rate at which this is trimmed back, and thus the equilibrium load, depends on the strength of selection for cognitive abilities)
Motivation: attention to learning, play, imitation, and language comes at the expense of attention to other things
Pleiotropy with other selection combined with evolutionary limits (selection for lower aggression also causes white patches in fur via changes in neural crests, and retention of a variety of juvenile features), e.g. selection for disease resistance changing pathways so as to accidentally impair brain function (with the change surviving because of its benefits)
Alleles that provide resistance to disease (genetic variance is maintained in a Red Queen’s Race) that damages the brain would be a source of genetic variance, likewise variants affecting nutrition or other environmental influences