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How Can There Still Be a Sex Difference, Even When There Is No Sex Difference?

Why men may be more variable on some traits.

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Last time we discovered that the brain of a woman is composed of two different types of cells. One type has an X chromosome inherited from her mother, and another type of cell has an X chromosome from her father. If we could see these different types of cells in a brain scanner, and assign each a different color, the brain of a woman would look like a patchwork mosaic of two colors -- rather like the fur of calico cats. A similar scan of a man's brain would show only one color, because all of his brain cells inherited the X chromosome from his mother.

This "genetic mosaicism" of women's brains may lead to an interesting sex difference that spans many different traits: men show more within-sex variability than do women. This means that there can be a sex difference between men and women on a trait even when their group averages are the same. Let's see how this could be true.

Here are the male and female distributions for a trait that is widely known to be sexually dimorphic -- height. The male group average for height in the U.S. is about 69" and this is substantially taller than the female average height of about 64". We can easily see that there is a sex difference on this trait.

But there are two ways that the sexes could differ. They could be different in average group scores, as above. Or, they could have the same mean score, but differ in the amount of variation on the trait for each sex. That is, there can be a sex difference in within-sex variability, even when there is no sex difference in between-sex average group scores.

In this photo of height by the sex of college students, we can clearly see that the group averages are different. But can you also see the within-sex variability difference in distributions?

Surprisingly, on many traits males are simply more variable than are females.

Here is another example. Below is a graph of IQ scores of boys and girls in Scotland. Note that there are more boys than girls at both the high and low ends of the distribution.

This finding of greater male variability in IQ scores has been replicated with many different populations and in more modern times. (See, for example, Hedges and Nowell (1995); and in particular see the appendix of Lubinski and Benbow (2006): Study of mathematically precocious youth after 35 years.)

You may recall that Larry Summers was forced to resign as President of Harvard University when many people simply misinterpreted his remark that males are more variable than are females on many traits:

It does appear that on many, many different human attributes-height, weight, propensity for criminality, overall IQ, mathematical ability, scientific ability-there is relatively clear evidence that whatever the difference in means-which can be debated-there is a difference in the standard deviation, and variability of a male and a female population.

Most of his critics misunderstood his remarks and presumed that he was suggesting that males are on average more intelligent than females. Or, if they understood him correctly, some may have found it interesting that there were more intellectually deficient males than females, but the sex ratio at the other tail of the distribution was less palatable. That is, they may have committed the morallistic fallacy -- the assumption that if something is morally objectable, either on its face or in its possible misinterpretation or misuse, it cannot be factually correct.

However, if it is simply a fact that males are generally more variable than are females on many traits, why is this true?

A possible explanation is based on the genetic mosaicism of females, as noted above. An X-linked gene inherited from the mother that coded for an extreme trait (either high or low) might be moderated in a female by an alternate allele inherited from her father that has a less extreme effect. However, a male who inherited an X-linked gene that coded for an extreme trait would have the same gene in all of his cells (because he doesn't have a second X chromosome) and thus there would be no moderating effect.

For example, ocular albinism (a genetic disorder of the retina) is a X-linked disorder. A woman who inherited this gene from her mother, but not her father, would have a retina with a combination, or a "mosaic," of clumps of normal cells (derived from the normal gene from her father) intermixed with clumps of non-functioning albino cells (derived from the defective gene from her mother). Although her vision would be deficient, she would not be blind. However, because a male with this gene would have all his retinal cells derived from the defective X-linked gene from his mother, all of his retinal cells would be albino, and he would be blind.

If some genes that code for unusual extremes on a trait are X-linked, males would inherit these only from their mothers -- they would not be moderated by non-extreme X-linked genes of the father. On the other hand, females who inherit an extreme trait from one parent might have the effects of that gene moderated by the non-extreme gene from the other parent. That is, extreme traits that are X-linked are "averaged" in women. Thus, for traits that are influenced by X-linked genes, males are likely to be more variable than females.

Female genetic mosaicism might also be related to another intriguing brain sex difference. Neural functionality seems to be more spread out in female brains. For example, if a male and a female have a brain injury (e.g., a stroke) that damages the same area of the brain, generally males will show a greater cognitive deficit. Why is cognitive function in male brains generally more localized, and in females it is more diffused over a larger area of the brain? We don't know, but the possibility that genetic mosaicism might also have some role to play in this sex difference is intriguing.

How the brains of males and females are different is called a proximate question. Another complementary question is an ultimate one -- why did male and female brains become different?

Here is a possible evolutionary explanation offered by Steven Pinker:

Since a male can have more offspring than a female--but also has a greater chance of being childless... natural selection favors a slightly more conservative and reliable baby-building process for females and a slightly more ambitious and error-prone process for males.

This leads us to another intriguing difference between the sexes, again, with males showing more variability than females. There is greater reproductive variance among males than among females. That is, there is more variability between males with respect to the number of children they have than there is among females. Reproduction is thus is a higher stakes game for males -- both the upside (the possibility of having more children than any woman could ever have) and the downside (the possibility of being involuntarily excluded from mating and reproduction entirely) are greater. This leads to some very interesting sex differences.

And we will explore that next time!

For further reading:

Hedges, L. V., & Nowell, A. (1995). Sex differences in mental scores, variability, and numbers of high scoring individuals. Science, 269, 41 - 45.

Lehrke, R. (1997). Sex linkage of intelligence: The X-Factor. NY: Praeger.

Lubinski, D., & Benbow, C. M. (2006). Study of mathematically precocious youth after 35 years. Perspectives on Psychological Science, 1, 316 - 345.

Copyright © Michael Mills

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