Hedges and Nowell (1995) write that

Sex differences in central tendency, variability, and numbers of high scores on mental tests have been extensively studied. Research has not always seemed to yield consistent results, partly because most studies have not used representative samples of national populations. An analysis of mental test scores from six studies that used national probability samples provided evidence that although average sex differences have been generally small and stable over time, the test scores of males consistently have larger variance. Except in tests of reading comprehension, perceptual speed, and associative memory, males typically outnumber females substantially among high-scoring individuals.

However, they don't really say anything about the reason for this aside from that "Our results shed little light on the origins of sex differences in [...] variability" (p45).

What theories are there of the reasons for this sex difference in variability on mental tests, and how strong is the evidence to support these theories?

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    $\begingroup$ Theories are similar to those explaining differences in intelligence between African American and Caucasian American samples, namely that socio-economic factors such as education, health, peer stimulation etc. play a role. (Feminist) theories hold that women are disadvantaged and that lower performance of women in certain areas is caused by this. Male children are less stimulated by their (mostly female) guardians in social and verbal areas and therefore score lower there. Wether you want to believe this is up to you. There is no experimental evidence, only conclusions drawn from observation. $\endgroup$ – user3116 Feb 25 '15 at 10:19

I've heard two theories related to genetics. Mental traits are massively polygenic (many genes with tiny effects). Men (XY) do not have a duplicate X chromosome like women (XX), which means that mutations to an X chromosome that disrupt typical gene expression can affect men more than women, driving variance in mental scores. The second theory is related to the fact that individual men are less likely to pass on their genes than individual women. Counter-intuitively, most of our ancestors are women. Men may have evolved a riskier genetic strategy that produces more high and low cognitive scores in phenotypes. Consider that men also take more risks. This is a behavioral outcome that may have emerged from the same selection pressure.

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