The "sex difference factor"?

By Razib Khan | January 5, 2012 3:10 pm

There’s a new paper in PLoS ONE, The Distance Between Mars and Venus: Measuring Global Sex Differences in Personality*, which suggests that by measuring variation of single observed personality traits researchers are missing larger underlying patterns of difference. The Distance Between Mars and Venus: Measuring Global Sex Differences in Personality:

In conclusion, we believe we made it clear that the true extent of sex differences in human personality has been consistently underestimated. While our current estimate represents a substantial improvement on the existing literature, we urge researchers to replicate this type of analysis with other datasets and different personality measures. An especially critical task will be to compare self-reported personality with observer ratings and other, more objective evaluation methods. Of course, the methodological guidelines presented in this paper can and should be applied to domains of individual differences other than personality, including vocational interests, cognitive abilities, creativity, and so forth. Moreover, the pattern of global sex differences in these domains may help elucidate the meaning and generality of the broad dimension of individual differences known as “masculinity-femininity”…In this way, it will be possible to build a solid foundation for the scientific study of psychological sex differences and their biological and cultural origins.


I’m curious about the reaction of people in psychology to this result. The reason is that I am generally confused or skeptical about measurements of personality difference. I’m not confused or skeptical of differences in personality between individuals or groups. I agree that these exist. I just don’t have a good sense of the informativeness of the measures of difference. People may criticize psychometrics intelligence testing all they want, but at least their methods are relatively clear.

From what I can gather the authors discovered that the differences between sexes on personality were much clearer once you looked for the correlation across numerous single measured traits. This strikes me as similar to what you see in population genetics when you move from variation in one gene across populations to many. While a single gene is not very informative in terms of population differences (e.g., the standard assertion that ~15 percent of variation is between races), synthesizing the variation of many genes allows one to easily distinguish populations, because there is such strong discordance in the correlation of differences. An analogy with traits makes understanding this easy. If you were told that population X tended toward black hair, that would not be very informative. Nor if you were told that population X tended toward straight hair. And what if you were told that population X tended toward light skin? All these traits are common across many different populations. But if you told that population X tended toward straight black hair and light skin, the set of populations which intersect at those three traits together in this direction is far smaller than evaluating on a trait-by-trait basis.

But in regards to the evolution of sex differences there is something that I feel that I can say here. Humans seem to lay between other ape lineages in terms of physical dimorphism. For example, in size the difference between males and females is not as extreme as gorillas, but not as equitable as among gibbons. These differences are traditionally correlated with social structure. Groillas are highly polygynous, and there is a great deal of male-male competition, therefore driving sexual selection. In contrast, gibbons tend toward monogamy (at least in the ideal, as with “monogamous birds” the reality seems to differ from the ideal).

But there is also an evolutionary genetic aspect to sexual dimorphism we must consider: in Genetics and Analysis of Quantitative Traits the authors note that evolution of sex specific traits is not going to occur fast. The reason is simple: aside from the peculiarities on the sex chromosomes males and females are genetically the same. This implies that sex differences on the genetic level may emerge via modulation of gene expression across networks of genes tuned by some “master controllers” associated with differential sex development. All of this added complexity takes time to evolve, with the rough result that sexual differences in trait value take about an order of magnitude longer than other traits to come to the fore. The intuition here is simple: if there is selection for large males, there will be selection for large daughters indirectly. Modifiers which dampen this effect need to emerge, so that sex-specific selection doesn’t have the side effect of dragging the other sex along in terms of trait value (this is a concern when you have traits, such as high testosterone, which might increase fitness in males, but reduce it their daughters). Therefore, if there are sex differences in behavioral tendencies which are biologically rooted (I believe there), they will tend to be universal across human societies and have a very deep evolutionary history.

So that would be the strategy to understand differences in personality across the sexes. Go beyond W.E.I.R.D. populations, as they did in this study. And look for traits where males and females seem to exhibit consistent differences across these range of social environments. I suspect environment does effect the magnitude of differences, but I would be willing to bet money that some differences are going to persist (e.g., inter-personal violence is an area where males will differ due to size and personality).

* I’m really sick of the use of the Mars vs. Venus dichotomy in the scholarship.

CATEGORIZED UNDER: Psychology
MORE ABOUT: Sex Differences
  • JL

    I am generally confused or skeptical about measurements of personality difference … People may criticize psychometrics all they want, but at least their methods are relatively clear.

    ?? Measurement of personality differences is psychometrics.

  • http://blogs.discovermagazine.com/gnxp Razib Khan

    #1, you’re right. i meant intelligence testing!

  • http://twitter.com/robsica Rob

    Critical response by Hyde: http://bit.ly/xSoNU7

  • Grey

    “the authors note that evolution of sex specific traits is not going to occur fast”

    Maybe a dumb question but couldn’t a simple way of getting around the problem you mention be traits that only pass on the X, or the modulator only passes on the X, so females automatically get a double-dose and men only one?

  • Paul Rain

    Grey: This would be very very unlikely to happen for mammals due to X-inactivation, where one of the X-chromosomes is disabled for each cell present in early embryonic development. Wiki has an explanation, but there’s a better one freely available at the Nature site.

  • Sister Y

    “Therefore, if there are sex differences in behavioral tendencies which are biologically rooted (I believe there), they will tend to be universal across human societies and have a very deep evolutionary history.”

    Agree that MOST would be deep/universal, but if the intensity of sexual selection varies by population, wouldn’t you expect some non-universal sex differences to evolve in populations exposed to high sexual selection? (Expanding the Peter Frost eye-and-hair-color variation thing to psychological traits.) http://cogweb.ucla.edu/ep/Frost_06.html

  • http://blogs.discovermagazine.com/gnxp Razib Khan

    but if the intensity of sexual selection varies by population, wouldn’t you expect some non-universal sex differences to evolve in populations exposed to high sexual selection?

    sure. but the model peter outlines is pretty dated. e.g., Yet skin color is weakly influenced by the different alleles for hair color or eye color apart from the ones for red hair or blue eyes. true in ~2005, but not true now. the pigmentation traits are strongly pleiotropic. re: sexual selection, it is usually due to variation on males. that should show up diff. sex effec. populations in different populations.

  • immu

    Razib, you should read more about personality psychology. Personality is now studied in all the animals, and it includes all the behavioural differences between individuals. Modern personality measures have been under development for decades and they are quite reliable. Extensive factor analysis has been done to squeeze a small number of factors from thousands of adjectives describing personality. Thus, peronality measure development resembles iq test development in many ways.

  • IW

    “I’m really sick of the use of the Mars vs. Venus dichotomy in the scholarship.”

    I blame Paul McCartney. They were going to use Neptune and Uranus, but whilst one of those is blue, the other isn’t pink, and both are way too nebulous….

  • John Roth

    I’m not sure I see the similarity between this and the inter-population differences in genetics. The latter is fairly straight-forward. Differences in specific genes are usually binary and there is no connection to either the biology or phenotype. The connection with inter-population differences is fairly clear with genetic drift theory.

    Personality is a phenotypic measurement that’s disconnected from the underlying biology. Without that connection it’s impossible to say what we’re looking at with any degree of assurance. Contrary to the Hyde reference in @3 Rob, I have no great confidence that currently accepted measures are going to turn out to be fundamental once the genetics to biology to phenotype connections are worked out. The latter is, of course, proceeding at a glacial pace.

    For the record, I think there are significant differences.

  • http://washparkprophet.blogspot.com ohwilleke

    “I’m not sure I see the similarity between this and the inter-population differences in genetics.”

    What is similar is the kind of statistical analysis that is done and the way that the choices of statistical methodology influence the power of the statsitical inferences that can be drawn from the data.

  • http://blogs.discovermagazine.com/gnxp Razib Khan

    Razib, you should read more about personality psychology

    anything specific?

  • Emma

    I totally agree that this is similar to the situation in population genetics.

    At each variable, there are only small differences between the groups (sex or populations). Therefore using only one variable produce a lot of overlap between the distributions of the two groups, but using many variable simultanesouly allows for a good separation between the two distributions. The authors equate little overlap with big effect, but I do not find that very convincing. In my opinion, when you have little differences for many variables, if you want to estimate the global effect it feels more correct to “average” the differences than to “add” them, which is what the authors did.

    Regarding personality, there was a nice themed issue on “evolutionary and ecological approaches to the study of personality” in Philos Trans R Soc Lond B Biol Sci, dec 2010

  • observer

    #10

    I guess I would see the point being quite the opposite. If we care about anything in the ordinary conduct of our lives, it would be differences in phenotype, not differences in genotype. In fact, the sort of differences attributable to genetic drift may well have no discernible impact on phenotype, even if they allow us to differentiate population groups quite well on a genetic level.

    But we certainly generally care about personality differences.

    How to understand the differences found in this paper is not entirely clear to me. I’m sympathetic to the criticism that the particular latent variables upon which the distinction between genders is most pronounced don’t seem to correspond to any trait we find independently meaningful. On the other hand, personality is a complex entity in any case, and it’s significant that we seem to be able to separate pretty effectively male from female personalities if we take all attributes into account.

  • Grey

    @Paul Rain, ty

  • Richard Sharpe

    “For example, in size the difference between males and females is not as extreme as gorillas, but not as equitable as among gibbons.”

    I suspect you really should use “similar” above rather than “equitable” because its definition in one dictionary I checked does not seem to be what I think you mean.

  • http://emilkirkegaard.com Emil

    “(e.g., the standard assertion that ~85 percent of variation is between races)”

    shudnt this be?

    “(e.g., the standard assertion that ~15 percent of variation is between races)”

    or

    “(e.g., the standard assertion that ~85 percent of variation is within races)”

  • immu

    My background is cognitive science, and what I noticed was that most people ignore personality too much. Which is strange as personality is as important as intelligence in many ways – and even more interesting as it seems so that different strategies and personalities have been there for millions of years. So, just some basic courses of personality psychology for everyone. I recommend Robert Sapolski’s lectures from youtube to everyone – the course will teach you a lot old and new about the relationship between biology and psychology.

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This blog is about evolution, genetics, genomics and their interstices. Please beware that comments are aggressively moderated. Uncivil or churlish comments will likely get you banned immediately, so make any contribution count!

About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at http://www.razib.com

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