The answer is: They aren’t.
The orthodoxy in intelligence research for the second half of the 20th century had been that men and women had the same average intelligence, but men had greater variance in their distribution than women. Most geniuses were men, and most imbeciles were men, they said, while most women were in the normal range. This conclusion, however, was manufactured out of political expediency. Not wanting to discover, or a priori denying, any sex differences in intelligence, psychometricians simply deleted from the standardized IQ tests any item on which the performance of men and women differed.
More recently, however, especially since the turn of the millennium, there have been an increasing number of studies that cast doubt on this politically correct conclusion. Studies with large representative national samples from Spain, Denmark, and the United States, as well as meta-analyses of a large number of published studies throughout the world, all conclude that men on average are slightly but significantly more intelligent than women, by about 3-5 IQ points. So this has now become the new (albeit tentative) consensus in intelligence research.
However, these studies do not answer the ultimate evolutionary question of why men should be more intelligent than women. General intelligence likely evolved as a domain-specific psychological mechanism to deal with evolutionary novelty. However, unlike populations in different geographic parts of the world, men and women within a population have always faced the same level of evolutionary novelty throughout evolutionary history, because they have always migrated together. If general intelligence is a function of the evolutionary novelty of the environment, why then are men on average slightly more intelligent than women?
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.
So it is not that men are more intelligent than women, but that taller people are more intelligent than shorter people, but net of height women are more intelligent than men. Women who are 5’10” are on average more intelligent than men who are 5’10”, and women who are 5’5” are on average more intelligent than men who are 5’5”. But, more importantly, people who are 5’10” are significantly more intelligent than people who are 5’5”, and most people who are 5’10” are men and most people who are 5’5” are women.
This conclusion simply leads to another question: Why are taller people more intelligent than shorter people? I’ll address this question in my next post.
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