The Greater Male Variability Hypothesis
How sex differences can be large even when they’re small, Part 2
This is the ninth excerpt from my forthcoming book, A Billion Years of Sex Differences. You can access the full collection here, and preorder the book here.
In this installment, I introduce the greater male variability hypothesis: the controversial idea that, for many traits, males vary more than females. One implication of this is that sex differences can be modest or even absent at the mean, yet large at the tails of the distribution.
The excerpt is fairly brief, so I thought I’d make it a freebie. Hope you enjoy it!
How Sex Differences Can Be Large Even When They’re Small
2. Men and Women Sometimes Differ in How Variable They Are
There’s another reason that sex differences can sometimes be larger at the extremes than the mean. For some traits, one sex is more variable than the other – that is, the peak of the distribution is slightly lower for that sex, while the tails stretch out slightly further on either side. Most often, males are more variable, an observation that goes by the name the greater-male-variability hypothesis. What this implies is that even if the average scores for a trait are identical for both sexes, somewhat more men than women will be found at both extremes, while somewhat fewer men will be found in the centre (although to be clear, most will still be found in the centre; it’s a subtle effect).
Greater male variability has been documented for a wide range of traits, including height, running speed, various brain parameters, daily energy expenditure, grades, aggression, risk-taking, some mate preferences, some personality traits, some occupational preferences, specific cognitive abilities such as verbal, maths, and spatial ability, and even general cognitive ability or intelligence.
The greater-male-variability hypothesis is controversial, and the subject of vigorous debate. Enemies of the hypothesis point out that variability sex differences are often trivially small, and that some studies don’t even find them. They also point out that, for some traits, women are more variable than men. (Curiously, greater female variability often centres on traits related to sexuality, including orgasm frequency, sex drive, and sexual enjoyment.) Still, the hypothesis has enough research behind it to justify taking it seriously. And to the extent that either sex exhibits greater variability, this may be an additional contributor to larger sex differences at the extremes than in the general population.
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