There is one argument against the claim that the distribution of intelligence is different by race or gender that I have very rarely seen and have not discussed here — the claim that since there is no advantage to being stupid there is no reason why evolution would produce any stupid people. The obvious problem with it is that, race and gender aside, stupid people exist. But that raises the question of this post. Being smart is a good thing, it is possible to be smart — some people are — so why isn't everybody?

I see three possible sorts of explanation.

Biological Cost

The brain is expensive, uses a much larger fraction of energy than its fraction of body weight. I don't know if there is evidence that the cost is larger for smarter people, don't even know if there is a correlation between brain size and intelligence, but both seem plausible. If so, there is a tradeoff between intelligence and nutritional requirements. Greater intelligence might have others sorts of biological cost, increase the risk of cancer say, or rate of aging. If, as seems likely, the reproductive advantage of intelligence faces diminishing returns, is less the more smart people there are, one would expect an equilibrium where the marginal benefit of increased intelligence was just balanced by the marginal cost.

Reproductive Cost

The reproductive advantages of increased intelligence are obvious: The smarter you are the better job you can do of producing food and evading predators. The smarter a man is the more able at persuading women to bear his children. The smarter a woman is, the more able to attract a high quality mate who will help to support the children she bears, avoid being seduced by males who won't.

But there is also a reproductive cost — because there is a conflict of interest between me and my genes. They "want" me to produce and successfully rear as many children as possible. I want to have a long and happy life. They try to bribe me into obeying them by making sex pleasurable. I take the bribe and escape obedience by masturbation, non-reproductive sex, contraception. They try to bribe me by making me enjoy interacting with children. I get a cat or dog to interact with instead. The more intelligent I am, the more likely I am to perceive the conflict and act to achieve my goals, not theirs.