I.Q and Human Intelligence

Fig. 1. Total sample male and female general intelligence g (HFA/S–L, see text) distributions and male–female ratio as a function of male g = .23 (SD = 1.03) and female g =
.23 (SD = .93). Nmales = 90 (mean age 13.0, SD = 3.54) and Nfemales = 91 (mean age 12.8, SD = 3.6).

In Detail

The brain size of adult men is about 110 cc (0.78 SD) larger. The IQ-brain size correlation is +0.44, so the theoretical male superiority in adulthood is (0.78×0.44) = 0.35SD, ie in IQ points: (0.35×15) = 5.1 IQ points, exactly what is found empirically in the tests.

Development

A problem that has sometimes been raised as part of the positive relationship between brain size and intelligence is that women have smaller brains than men and yet it has been virtually universally claimed that there was no differences of intelligence between men and women.
For example, “women’s brains are 10% smaller than men’s, but their IQ is on average the same” (Butterworth 1999: 293). Since women with a smaller brain size would be as smart as men, it appears that brain size has no effect on intelligence. This is the conclusion drawn by Gould (1996, p.132), who writes that he refutes “the myth that differences in brain size would have a relationship to intelligence” The smaller average size of women’s brains has been demonstrated by Ankney (1992) and Rushton (1992). Ankney has calculated that the average men’s brain, adjusted for body size, is 100 grams heavier. Rushton calculated from another set of data from 6325 military men that the brain average, adjusted for body size, is 1442cc for men and 1332cc for women, a male advantage of 110cc; 1CC of brain tissue weighs about 1 gram, so that the results of Ankney and Rushton are very similar.

So we have a paradox: the size of the brain is positively related to intelligence, men have on average a larger brain, and yet men and women would have the same intelligence.
Richard Lynn presented the resolution of this paradox in Lynn (1994 and 1999) and in Lynn and Irwing (2004).

Up to the age of 15 years, males and females have approximately the same intelligence, except for a small male advantage on the visualization abilities; however, from the age of 16 years, males begin to show greater intelligence, reaching an advantage of from three to five IQ points in adults. This has been further confirmed by Paul Irwing and myself (Irwing & Lynn, 2005; Irwing, 2012), by Victoria Bourne, Helen Fox, Ian Deary, and Lawrence Whalley (2007).

This advantage is fully explicable by the larger size of mens’ brain. Men and women experience the same environment and therefore environmental factors can not account for the difference between men and women. The evolutionary  explanation of the average slightly higher intelligence of men is that men compete with each other to obtain women and in the evolution of hominids, the intelligence played an important role. Women do not compete for men.

Coming to corroborate male intellectual advantage, Deary has shown that men have a faster reaction time (RT) than women. This is a consequence of the positive correlation between simple reaction times and Q.I.

A new study (2019) on 10,300 Spaniards again confirmed the difference in speed of development between the sexes, with a small male cognitive advantage in adulthood.

In adulthood, men’s I.Q is on average slightly higher (4-5 I.Q points).

Below, summary of sex differences found at the national level. A minus sign indicates a female advantage (only in Argentina). For all studies, the male intellectual advantage is 0.206SD, ie 3.1 points of Q.I.

Summary on Sex Differences in Intelligence by Richard Lynn (2017).

Click to enlarge