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Male brains are very much bigger, a colossal 1.4 effect size. 92% of men will be above the mean for women. On average men have 117.8 cm3 more brain than women. All this extra brain must be doing something for men, you might surmise, other than dreaming about sexual intercourse.

— James Thompson (@JamesPsychol) March 5, 2019

So what does it mean? Are 92% of men really smarter than the average woman?


Mr. Thompson explains,

However, cognitive tests show an effect size of only 0.2 in favour of men, just 3 IQ points. In consequence, at IQ 130 70% are men and at IQ 145 80% are men.

It is possible that, in a rush to ensure that men and women’s mental ability scores can be presented as equal, men’s stronger subject areas have been under-sampled. Test producers are under pressure to minimize sexual and racial differences.

In defence of any group who think that their specialist strong points have been ignored, we should set the sampling frame for cognitive tests as wide as possible. There may be a small but real male advantage in intelligence which a broader scope of tests would reveal.

Bigger brains are associated with bigger bodies, but the very large male-female brain size discrepancy suggests something else is going on besides correlated brain-body size dimorphism.

From Thompson’s article on the study over at Unz:

Broadly the same effect of male advantage can be found in all the brain region sub-comparisons. Male brains are both larger, and also vary more in size. Greater male variability seems a fact of nature. If there were a direct relationship between brain size and cognitive ability, there would be many, many more bright men than bright women.

The cognitive test was limited to a 13-item verbal-numerical test to be completed in 2 minutes, which ought to be enough to grade the general population.


The test might be a little crude if the purpose is to detect sex differences across the broad range of different cognitive tasks, and also a bit limited if the volunteers are, as one might expect of this database, somewhat brighter volunteers interested in contributing to science. However, these are minor quibbles. All intellects can be evaluated in 2 minutes.


Sure enough, Table 2 shows that the cognitive tests are only an effect size of about 0.2 in favour of men. Where did all the male brain size advantage go? 0.2 of a standard deviation works out to 3 IQ points. Nothing much, you may say, considering that the test-retest reliability of the Wechsler is 4 IQ points, but if this is a true representation of male-female differences, then we can calculate what it would mean for the male/female balance at the higher levels of ability. As you may have seen in previous posts, if men are really 3 points brighter than women, and women’s standard deviation is narrower than men, say 14 rather than 15 points, then this makes a big difference at the higher reaches of intelligence.

Here are the estimates, if one assumes men have an IQ of 102, (sd 15) and women an IQ of 99, (sd 14).

At IQ 130: 69.8% men
At IQ 145: 80.3% men


This is a very substantial paper. It shows a massive sex difference in brain size of 1.4 d, and when one factors in that brain size relates to intelligence at a correlation of about 0.28, then the predicted intelligence difference will be a large 0.39 d, but the observed difference is only half that. Paradoxical. One implication is that there are sex-linked differences in brain structure and dendritic arborization which overcome pure size differences. If so, how is this balancing act achieved? Why don’t all people have the smaller, more craftily wired version of the human brain, which presumably requires a smaller blood supply. On the other hand, it might be that the cognitive testing has not been wide enough, and has ignored tasks in which males have an advantage. By the way, if one sex has an advantage in one skill, this is not an error of testing, it is a triumph of testing that a real difference has been revealed.

Speculatively, it’s not just cognitive tests that could be ignoring tasks which favor men. Colleges could be selecting students based on criteria that likewise ignore areas in which men have an advantage.

In honor of International Whambamthankyoumaam Day, here is a picture of misogyny:


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