In my previous post
, I explain that men on average are slightly but significantly more intelligent than women, not because they are men, but because they are taller. But why are taller people more intelligent than shorter people?
The real answer is we don’t know for sure, but there are two possible explanations. First, both height and intelligence may be indicators of underlying health. According to this view, people who are genetically and developmentally healthier simultaneously grow taller and become more intelligent than those who are less healthy, producing the positive correlation between height and intelligence.
This is a plausible theory. In our paper, however, Reyniers and I produce evidence against it. In the analysis of the National Longitudinal Study of Adolescent Health, we control for the respondent’s health, by using four indicators of health and producing a latent variable for health using principal component analysis in order to eliminate random measurement error. The association between height and intelligence does not diminish at all when we control for health. In fact, once we control for other demographic and social variables, health is not significantly correlated with intelligence at all; it actually has a nonsignificantly negative association with intelligence. So, at least in our sample, health is unlikely to be the common cause for both height and intelligence.
In our paper, Reyniers and I propose a second possible explanation. It consists of three separate mechanisms.
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.
We believe that this may be why taller people are more intelligent than shorter people. Another factor contributing to the seeming male advantage in intelligence is that taller parents are more likely to have sons than shorter parents. So, over many generations, more sons will inherit their parents’ genes inclining them to be taller and more intelligent, and more daughters will inherit their parents’ genes inclining them to be shorter and less intelligent. But, once again, the crucial factor is height, not sex.
In our paper, we present evidence for all of the crucial mechanisms: Taller people are on average physically more attractive than shorter people; physically more attractive people are on average more intelligent than physically less attractive people; taller people are on average more intelligent than shorter people; and taller parents are more likely to have sons than shorter parents. But the issue is far from resolved. While there is no doubt that taller people are indeed more intelligent than shorter people, the question of why this is so is one of the remaining puzzles in evolutionary psychology.