Sex differences in intelligence

© 2003-2020 Paul Cooijmans


In this discussion of sex differences I rely mostly on chapter 13, "Sex differences in g", from Arthur Jensen's book "The g factor", and a little bit on chapter 4, "Conditions for excellence", from Hans Eysenck's book "Genius - The natural history of creativity". Also on Richard Lynn's home page, and on my own experience with high-range I.Q. tests. The remarks regarding the effect of hormones on mental abilities are based on the review Testosterone levels in healthy men and the relation to behavioural and physical characteristics: facts and constructs by M. Zitzmann and E. Nieschlag (which, in turn, refers to a large number of studies in that field).

Mean I.Q.

When it comes to the question whether or not there is a difference in mean I.Q. between males and females, Jensen basically says no, after having considered a large amount of evidence (more precisely, Jensen is referring to the difference in g rather than in I.Q.). Eysenck is a little bit more sceptical and points out that the usual assumption of equal I.Q. of the sexes may be flawed. Based on data also mentioned by Jensen (R. Lynn, 1994, "Sex differences in intelligence and brain size: a paradox resolved"), Eysenck suggests 4 I.Q. points as a conservative estimate of the difference (favouring males). Lynn, on his home page, simply states in adults the difference is about 5 points.

Both Jensen and Eysenck indicate that the question is hard to answer, as I.Q. tests like Stanford-Binet and WAIS have traditionally been constructed to show no sex difference in total score, by leaving out or counterbalancing items that show sex differences. Such tests therefore are not capable of measuring a possible difference between the sexes.

For better understanding, one should know that I.Q. tests measure not only g (the common factor shared by all tests for mental abilities), but that about one third of the variance in I.Q. is due to group factors (e.g. verbal, numerical, spatial, memory) common to some but not all tests, and to test specificity. Therefore a sex difference in mean I.Q. does not imply a sex difference in g, as some or all of the difference may lie in non-g factors.

I myself can not observe a mean difference directly as I only deal with high-range tests. There is a statistical report on sex differences on high range I.Q. tests, which is relevant regarding this matter.


The male variance in I.Q. is greater than that for females; Jensen says this difference is greatest in mathematics and spatial ability. In mathematics the male variance is 1.1 to 1.3 times greater. He concludes from this and from various other facts that the cause of the greater male variance in I.Q. lies mainly or entirely in non-g factors rather than in g.

In the high range, my own observation to date is that at or above the 98th percentile there are about twice more males than females, while at or above the 99.9th percentile there are about 15 times more males. These estimates are based on the male/female ratios in certain high I.Q. societies and on analysis of male and female performance on my tests. Trying to make this fit in terms of standard deviation, I find that when the male and female mean are both I.Q. 100, the male standard deviation must be about 33% greater than the female standard deviation. However, if a mean difference of 5 points in favour of males existed, the male standard deviation would only need to be about 11% greater. I do not know which is true (or if the truth lies in between). I must say though that a difference of 33% seems unlikely.

A remark sometimes made regarding the male/female ratio in I.Q. societies and among high-range candidates is that females may be so lowly represented because they simply do not like taking intelligence tests, or even because they attach less value to intelligence than males do as a result of having been raised and socialized to value other traits higher in women. In other words, that their representation is not proportional to their actual presence at high intelligence levels. The following facts, however, speak against this:

These facts make it almost inescapable to conclude that the male/female ratio among high-range candidates and in I.Q. societies is roughly representative of the actual male/female ratio at high intelligence levels. Sometimes one has to leave a prejudice (equality) behind and accept the facts, even if one would like them to be otherwise. That I say "roughly representative" is not because I believe that "liking tests" or social or socialization factors may still contribute, but to leave open the possibility that yet other personality features are at play next to intelligence, like associative horizon.

Childhood and growing up

Girls mature earlier verbally, and after the onset of puberty boys catch up. The male advantage on spatial and numerical ability (discussed further on) is not yet present in young children, and develops slowly during childhood and puberty. Important to realize here is that at least some of the sex differences in mental abilities are likely caused by hormonal differences (oestrogen/testosterone balance), which work partly prenatally and partly after the onset of puberty, but are absent in childhood.

If there is a mean difference in I.Q. between the sexes, this will be fully expressed only in adults, and not yet in children. In any case, it seems that when testing children, e.g. for "giftedness", one should be aware of these developments and differences, the risk being that one selects too many girls and too few boys as "gifted".


Women suffer from dementia more often than men, and deteriorate more rapidly when suffering from Alzheimer's disease (which is one form of dementia) compared to men who are in the same phase of the disease. This difference can not be explained by the mere fact that women become older than men. One suspects that hormones play a role, and the male brain appears to be better protected against the damage of a disease like Alzheimer. Since dementia is basically a decline of mental ability (so, of intelligence) this difference in rate of dementia must be considered a sex difference in intelligence.

Ability types at which females are better than males

Females are slightly better than males at straightforward arithmetic (not at more complex mathematics). On short-term memory the difference is greater; they score .3 σ (standard deviation) higher than males.

A verbal ability type that consistently favours females is "fluency"; such tests require the candidate to name as many as possible words starting with a given letter within a limited time. Females are also better at reading, writing, grammar, and spelling. The popular notion that females are better than males at verbal ability on the whole is not true; they are only better at these specific tasks, while there is no or as good as no sex difference in verbal ability on the whole (the notion of females being better at verbal ability may be related to the popular confusion of talkativeness with verbal ability, and of "verbal" with "oral"; for clarity, "verbal", in psychometrics, simply means "relating to or in the form of words", and not "spoken rather than written", which is a secondary meaning of the word).

Other tasks at which females outscore males are those involving perceptual speed (e.g. matching figures) and clerical checking, both speed and accuracy (e.g. underlining certain letters in a text, or digit/symbol coding). Their advantage on such tasks varies from .2 to .4 σ. Females are also better at motor coordination and finger and manual dexterity, but those are not mental abilities in a strict sense, although they do feature in the "performance" sections of some individual tests.

Ability types at which males are better

The largest single difference is that in spatial ability; the mental manipulation of figures in two or more dimensions. The difference varies from .3 to .5 σ. Studies link this difference to prenatal testosterone levels. A sex difference in spatial ability is also found in some animals, which suggests it is a more general biological phenomenon that predates the human genus.

Then there is a difference in numerical ability (except for simple arithmetic) of .1 to .25 σ and as already said, in both spatial and numerical ability there is also a large difference in variance, favouring males. When spatial and numerical ability (especially mathematics) are combined, the mean sex difference becomes much greater still than are the separate differences on spatial and numerical ability.

As for verbal ability, males are better at tests of general knowledge. In verbal reasoning there is as good as no difference.

Effects of altered sex hormone levels on mental abilities

When the testosterone/oestrogen balance changes drastically in an individual, such as through medical treatment, this has been found to affect visual-spatial ability and verbal fluency in exactly the ways one would expect, given the known sex differences in those fields: Testosterone treatment raises visual-spatial ability and decreases verbal fluency, while a reduction of testosterone decreases spatial ability. Oestrogen treatment increases verbal fluency. These effects may be taken into account when one considers undergoing such treatment.

The effects on general intelligence are not reported, but considering that males seem to have a wider spread of I.Q., and considering that spatial ability and verbal fluency are affected as can be expected based on known sex differences, an educated guess can be made: If one is near the average in general intelligence, drastically altered sex hormone levels may not change one's general level significantly, but mainly affect one's profile. If one is well above average or even highly intelligent, a change from a male to a female hormonal balance should logically lower one's I.Q. toward the average, while the opposite change should raise one's I.Q. A warning seems appropriate: persons who are very far above the average in intelligence tend to underestimate their distance to the mean or even consider themselves merely average. In matters like this, that might be a fairly expensive mistake.


Three vital points follow from the above:

  1. Because of the difference in variance (and regardless of the fact whether there is a mean difference), in any above-average sample it is normal to find adult males scoring higher than adult females, provided the test ceiling is high enough to allow the males to outscore the females, and the test has not been constructed to be sex-balanced rather than psychometrically sound (and, of course, provided the statistics have not been tampered with or concocted to produce a socially desirable outcome; such fraud is by no means uncommon in the social sciences).
  2. The state of affairs among children, regarding mental abilities, is not representative of that among adults. In childhood, girls appear deceptively equal or superior to boys. As a result, studies of mental ability in children or teenagers are not informative as to the sex differences among adults, nor do they prove that such differences have cultural rather than biological causes. On the contrary, the fact that the adult differences are shaped during and after puberty points to a biological cause: the testosterone/oestrogen balance. The sex hormones have begun their work early on in the womb, and continue from the onset of puberty onward. The period before puberty is one wherein the sex hormones are not active, which may explain the different relation between male and female ability levels in that period.
  3. When considering treatment that will change one's sex hormone levels, one should acquaint oneself with the possible effects on one's mental ability level and profile.

A dilemma that comes forth from these facts: Should "giftedness" be defined within children, or within adults? I am inclined to say "Beyond doubt within adults" to this question.