The male advantage in general intelligence
does not emerge until after puberty
, because girls mature faster than boys.
In an earlier post, I discuss the new consensus in the intelligence research in the 21st century that men on average are slightly (but significantly) more intelligent than women, by about 3-5 IQ points. However, in the same post, I also note that it is not because they are male that men are more intelligent but because they are taller. Taller individuals are more intelligent than shorter individuals, and men just happen to be taller than women. In fact, once we control for height, women on average are more intelligent than men. It still remains true, however, that, without controlling for height or anything else, if you simply compare men and women, men on average are slightly more intelligent than women.
Another little-known fact is that, because girls on average mature faster than boys, the male advantage in intelligence does not appear until after puberty, when boys and girls finish maturing and growing. Until then, girls are on average always more mature than boys at any given chronological age. So comparing boys and girls, say, at age 10, is like comparing boys at age 10 and girls at age 12. Naturally, older and more mature children have greater cognitive capacity than younger and less mature children. So if you compare boys and girls at the same chronological age, girls on average are more intelligent than boys. In other words, the sex difference in the rate of maturity masks and drowns the sex difference in general intelligence.
Here’s a perfect demonstration, using data from the National Child Development Study (NCDS) in the United Kingdom. The NCDS contains a population (not a sample) of all babies born in Great Britain (England, Wales, and Scotland) in one week in March 1958 (n = 17,419), and has followed them throughout their lives for more than half a century. The NCDS also has one of the best measures of general intelligence in all of survey data. They measure intelligence at age 7 (with four different cognitive tests), at age 11 (with five different cognitive tests), and at age 16 (with two different cognitive tests). Note that the respondents are largely before puberty at ages 7 and 11, but largely after puberty at age 16.
Here are the graphs that chart the mean IQ of the NCDS respondents by sex at ages 7 and 11, before puberty. You notice that girls are slightly but (given the large sample size) statistically significantly more intelligent than boys at both ages. At age 7, the mean IQ for girls is 100.6 while the mean IQ for boys is 99.4. At age 11, the mean IQ for girls is 100.4 while the mean IQ for boys is 99.6.
However, the sex difference is reversed at age 16, as the following graph shows. Post puberty, the mean IQ for girls is 99.2 while the mean IQ for boys is 100.8. Remember, these are the same individuals who are tested at three different ages in their lives. (And, no, it does not mean that girls become less intelligent after puberty in any absolute terms. The IQs are calculated and normed at each age separately. It only means that girls become less intelligent relative to boys after puberty.)
Because of their faster rate of maturity, girls are more intelligent than boys until puberty, but the male advantage in intelligence emerges when boys are fully mature and become taller than girls. (You notice that girls tend to be taller than boys at any given age until puberty.) Now the next evolutionary question is: Why do girls mature faster than boys? If you have been a regular reader of my blog, if you have been keeping score at home, you should be able to figure out the answer. (Hint: It’s the same reason that women are on average shorter than men.) But I will leave it for another post.