I came across two interesting posts by Satoshi Kanazawa on the fact that men on average have a slightly higher IQ than women. Here is a brief summary on the difference in average IQ. Kanazawa argues that men don’t on average have greater g than women because they are men, but simply because of their height. He argues:
. . . My LSE colleague, Diane J. Reyniers, and I offer one possible explanation in our article, forthcoming in the American Journal of Psychology. Psychometricians have known since the end of the 19th century that height is positively correlated with intelligence: Taller people on average are more intelligent than shorter people. And men in every human population are taller than women. So one possibility is that men are more intelligent than women, not because they are men, but because they are taller.
Our analysis of a large representative American sample from the National Longitudinal Study of Adolescent Health shows that this is indeed the case. In fact, once we control for height, women are slightly but significantly more intelligent than men. Further controlling for health, physical attractiveness, age, race, education, and earnings does not alter this conclusion. Height has exactly the same effect on intelligence for men and women: Each inch in height increases the IQ by about .4 point. The partial effect of height on intelligence is more than three times as strong as the partial effect of sex . . .
Of course, the next question to ask is: “How is height connected with IQ?” Kanazawa hypothesizes that three separate mechanisms come together to cause an increase in IQ re height:
. . . 1. Assortative mating of tall men and beautiful women. Because height is desirable in men and physical attractiveness is desirable in women, there should be assortative mating between tall men and beautiful women (and short men and less beautiful women). Because both height and physical attractiveness are heritable, this will create an extrinsic (non-causal) correlation among their children between height and physical attractiveness, where tall people (both men and women) are more beautiful than short people.
2. Assortative mating of intelligent men and beautiful women. Because intelligent men tend to attain higher status, at least in the evolutionarily novel environment in recent history, and high status is desirable in men, and because physical attractiveness is desirable in women, there should be assortative mating between intelligent (and thus high-status) men and beautiful women. Because both intelligence and physical attractiveness are heritable, this will create an extrinsic (non-causal) correlation among their children between intelligence and physical attractiveness, where more attractive people are more intelligent than less attractive people.
3. Extrinsic correlation between height and physical attractiveness (produced by Mechanism 1 above) and extrinsic correlation between intelligence and physical attractiveness (produced by Mechanism 2 above) will create a second-order extrinsic correlation between height and intelligence . . .
I can’t really comment on the merits of this hypothesis, but I do wonder about other factors such as brain size. Studies have shown in recent years that men on average have larger brains than women after controlling for body size. The larger brain size is specifically in grey cell matter found in certain areas of the brain contributing to general intelligence. There is a brief article on this here. The best reason for men having larger brains is that a larger brain better handles the complexity of hunting prey. Height can be related here, but I doubt causally. It seems to me that height and brain size will correlate with each other very well, and the common cause for this correlation will be successful hunting of prey.