Do men have more extreme variations than women?

Question

_{This question was considered unsuitable for Skeptics and I think it is more suited to BIology than Cognitive Sciences}

I was reading this article which I found interesting. It is not supported with references but was an invited address for the American Psychological Association.

It makes the claim that men go to more extremes than women.

People can point to plenty of data that the average IQ of adult men is about the same as the average for women. So to suggest that men are smarter than women is wrong. No wonder some women were offended.

There are more males than females with really low IQs. Indeed, the pattern with mental retardation is the same as with genius, namely that as you go from mild to medium to extreme, the preponderance of males gets bigger.

[...]

Almost certainly, it is something biological and genetic. And my guess is that the greater proportion of men at both extremes of the IQ distribution is part of the same pattern.

[...]

Nature rolls the dice with men more than women. Men go to extremes more than women. It’s true not just with IQ but also with other things, even height: The male distribution of height is flatter, with more really tall and really short men.

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Another notable example of a similar claim was Lawrence Summer's speech regarding the possible reasons for dearth of women in tenured positions in science and engineering at top schools.

Summers outlined 3 possible reasons, with a major kerfuffle erupting - resulting in his firing from the job as a Harvard University President - over his presented second reason [ quotes from Wiki ]:

The second hypothesis, different availability of aptitude at the high end, caused the most controversy. In his discussion of this hypothesis, Summers said that "even small differences in the standard deviation [between genders] will translate into very large differences in the available pool substantially out [from the mean]

Summers referenced research that implied differences between the standard deviations of males and females in the top 5% of twelfth graders under various tests. He then went on to argue that, if this research were to be accepted, then "whatever the set of attributes... that are precisely defined to correlate with being an aeronautical engineer at MIT or being a chemist at Berkeley... are probably different in their standard deviations as well".

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QUESTION: Are there any studies or evidence to support the claim that men have more extreme variations than women (in other words, that the trait's distribution in males have fatter tails) in a variety of traits?

Possible areas of research can be:

• Intelligence (general IQ or specific linguistic or spacial abilities)
• Physical attributes (strength, height, weight)
• personality traits
  • for lack of anything better, let's take the FFM's Five:
  • openness
  • conscientiousness
  • extroversion
  • agreeableness
  • neuroticism

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In order for the answer to be "yes", the research needs to show fat tails in at least 5-6 traits from all 3 of the buckets above (intelligence, physical, and personality), although precise trait mix may be arbitrary and not necessarily limited to my examples, as long as 5-6 somewhat independent traits fit.

In order for the answer to be "no", at least 8 of the 11 traits above should have no fatter tails in men.

Anything else means "inconclusive".

Answer 1

First of all for the first 22 chromosomes (and the mitochondrial chromsome) are the same between women and men. The X and the Y together are something like 1.5% of the total DNA in the human cell. The X chromosome is also in common, so it has the same chromosomal (genetic) variations the same as women and men.

While I am not sure about the differences you point out, its for sure there are lots of differences between the biology of men and women! The reason that women's traits might have different variances than men comes from the fact that in women there are 2 X chromosomes and in men there is only one, as well as the additional Y chromosome.

If there are any singular defects in the X chromosome a man carries, it will not be balanced out in the human male. (other organisms have different sex chromosome schemes). For instance because there is a cluster of genes that help the eye see in color, women's sense of color (i.e. from their cone cells) is sometimes better than men. With 2 X chromosomes there is even the possibility of a woman who has four different color receptors in their cones.

On the other hand, why men see better in the dark (i.e. greater sensitivity in the rod cells), is possibly due to an extra Blue Rod pigment gene that remains on the Y chromosome (see above reference). This is both an accident of genetics, but also an adaptive 'choice' that men would have different visual acuities than women.

Its also known that woman and men may have different epigenetic patterns, with some epigenetic modifications coming from the mother and others from the father. This is adaptive and also not the direct result of the XY chromosomes.

Interestingly, it turns out that for the most part women's 2 X chromosomes are mosaic. That is, in each cell only one of the randomly selected X chromosomes are active. Recessive/dominant X genes are often not the case.

This could result in a greater observed genetic variance for women because when you have two chromosomes, often one of the genes is completely silenced. Women display all of the alleles of their X chromsomes, whereas men have only one X chromosome entirely, the Y chromosome being much smaller.

So in other words, I would say that while men and women actually have the same variation in their DNA sequences, there are different patterns in the X and Y Chromosomes that might create more variance in terms of what we see in women's displayed genetic traits.

Answer 2

At least for physical data (heigth, weight etc) you can have a look of the DINED Anthropometric database http://dined.io.tudelft.nl/dined/. Here you can find mean and standard deviation data for Dutch population studies on numerous anthropometric measures, and you can stratify the results by sex.

Answer 3

The biggest problem with this question, (not just here, but for everyone who asks it,) is drawing on our culturally constructed definition of maleness to look for mechanisms of genetic inheritance of what are considered "male" traits. We could measure frequency distributions of height, but we don't have a way to accurately probe any of the other qualities you listed on a genetic level. Since we can't make these mechanistic connections, any conclusions we draw would be confounded by the nature vs nurture debate.

That said, I don't know of any studies to this effect. So this isn't much of an answer to your question.

EDIT: I've been thinking about this question all day.

The sources of heritable variation within genders that could be attributed to gender differences are (1) X and Y chromosome mutations and expression profiles, and (2) heritable epigenetic gender differences.

(2) might be more difficult to argue, since the currently known mechanism for epigenetic inheritance is the same in men and women.

One way to make this problem tractable is to look at transcriptome analysis -- it's an indirect approach to this question that gets from genome to phenotype quickly.

A cursory pubmed search led me to a study in Drosophila that takes a different approach to this question. They sought out previously undetected levels of gender difference. Most current studies (and there are a lot of them, including for humans) look to quantify differences in expression between genders for specific tissues. This is a prerequisite for estimating this within-genders variation, standardized against the number of genes expressed.

Answer 4

So far as we can tell any difference in variability (standard deviation) in humans is cultural not genetic and thus not consistent across the species.

You are talking about something called the Variability Hypothesis, which is most often referred to in intelligence. Variability hypothesis has long been criticised for being unsupported by evidence, or in the rare cases it has been supported, it is likely to be entirely cultural since it changes drastically with culture. It also quickly runs into the problem of defining intelligence, since different skills show different distributions with large influence by culture.

There have been no studies showing a difference in variability in any physical characteristics in humans across gender that I am aware of although males and females often have different means (height for instance). Keep in mind if men have a higher average height then their variability will appear to be higher just due to boundary effects (IE you cannot have negative heights)

In biology there is the Bateman's hypothesis that males will often show more variability than females, but this is unproven and largely untested. This study also shows numerous flaws in the original work. Don't get me wrong their are species where males show much higher variability in some characters but there are also plenty where females show higher variability, meaning this is likely a situational outcome and not a general principle.

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