If as your writings indicate, there is little or no sex difference in IQ, why do girls do better in school grades - even math/science (at least sometimes)? What sources would you suggest for reading on this subject., thanks, Jim

UPDATE May 2008

Sex differences in intelligence

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 of course on my own experience with high-range tests.

Note that "sex" here refers to one's state on the 23rd chromosome; females have two X chromosomes, males have XY. Some use the term "gender" in this respect, but that is incorrect as it refers more to one's identity on a feminine/masculine dimension, irrespective of chromosomal state.

When it comes to the question whether or not there is difference in mean IQ between males and females, Jensen basically says no, after having considered a large amount of evidence. Eysenck is a little bit more skeptical and points out that the usual assumption of equal IQ 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 IQ points as a conservative estimate of the difference (favoring males). Lynn, on his home page, simple states in adults the difference is about 4 points.

Both Jensen and Eysenck indicate that the question is hard to answer, as IQ 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.

I myself cannot observe a mean difference directly as I only deal with high-range tests. I will return to this point further on with regard to the variance difference.

The male variance in IQ is greater than that for females; Jensen says this difference is greatest in math and spatial ability. In math the male variance is 1.1 to 1.3 times greater (he does not give the difference for total IQ or 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. Trying to make this fit in terms of standard deviation (variance is the square of the standard deviation by the way), I find that when the male and female mean are both IQ 100, the male standard deviation (SD) must be about 33% greater than the female SD. However, when a mean difference of 5 points in favor of males would exist, the male SD would only need to be about 11% greater. I don't know which is true (or if the truth lies in between) and will not be able to verify it myself as I only deal with high-range tests. I must say though that an SD difference of 33% seems unlikely.

Girls mature earlier verbally, and after 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 the sex differences in mental abilities are caused by hormonal differences (estrogen/testosterone balance), which work partly prenatally and partly after puberty.

A remark I have on this: if there is a mean difference in IQ 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 would select too many girls and too few boys as "gifted".

Females are slightly better than males at straight-forward arithmetic (not at more complex math). On short-term memory the difference is greater; they score .3 SD higher than males.

A verbal ability type that consistently favors females is "fluency"; such tests require the testee 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're only better at these specific tasks, while there is no or as good as no sex difference on verbal ability on the whole.

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 SD. Females are also better at motor coordination and finger and manual dexterity, but those are not mental abilities in a strict sense.

The largest difference is that in spatial ability; the mental manipulation of figures in two or more dimensions. The difference varies from .3 to .5 SD.

Then there is a difference in numerical ability (except for simple arithmetic) of .1 to .25 SD. And as already said, in both spatial and numerical ability there is also a large difference in variance, favoring males.

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

Most important to realize is that because of the difference in variance (regardless if there is a mean difference), in any above-average sample it is normal to find males scoring higher than females. It would in fact be suspect to find this not to be the case; that would indicate possible problems with test ceiling (too low) or test construction (items selected to be sex-balanced instead of for psychometric soundness), or fraud with statistics.

A result of the greater male variance is that the correlations between specific high-range tests will be based largely on male testees, and that therefore the possible general factor in such tests is not the same as that in regular tests; in other words, high-range tests do not per se measure "g". If anything, they measure "male g".

Also important to know is that the state of affairs in children, regarding mental abilities, is not representative of how it will be when they have become adults.

Dilemmas that come forth from these facts: should test scores in the high range be expressed within-sex rather than sex-combined, given the difference? And: should "giftedness" be defined within children, or within adults? Think about it.

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