December 14, 2019


Taking Sex Differences in Personality Seriously: New approaches are shedding light on the magnitude of sex differences in personality and the results are so strong and pervasive that they can no longer be ignored. (Scott Barry Kaufman, December 12, 2019, Scientific American)

Personality is multidimensional, which has implications for calculating sex differences in personality. Relatively small differences across multiple traits can add up to substantial differences when considered as a whole profile of traits. Take the human face, for example. If you were to just take a particular feature of the face-- such as mouth width, forehead height, or eye size-- you would have difficult differentiating between a male face and a female face. You simply can't tell a male eyeball from a female eyeball, for instance. However, a look at the combination of facial features produces two very distinct clusters of male vs. female faces. In fact, observers can correctly determine sex from pictures with greater than 95% accuracy [4]. Here's an interesting question: does the same apply to the domain of personality?

Interestingly, yes. You can calculate a metric called D which is a summary of how statistically separate two groups are from each other (i.e., how good of a line you can draw between groups from a statistical point of view). This metric allows you to take into account how all of the personality traits tend to be related to each other in the general population. For instance, people who are conscientious also tend to be more emotionally stable, so if you find someone who is very conscientious and also super neurotic, that person stands out more (has a more unusual personality profile) given the overall correlational structure. With more traits, things get even more interesting. You can have a combination of traits that are less expected, and thus more informative, because they go against the trends of the correlational structure [5].

There now exists four large-scale studies that use this multivariate methodology (see here, here, here, and here). All four studies are conducted cross-culturally and report on an analysis of narrow personality traits (which, as you may recall, is where most of the action is when it comes to sex differences). Critically, all four studies converge on the same basic finding: when looking at the overall gestalt of human personality, there is a truly striking difference between the typical male and female personality profiles.

Just how striking? Well, actually, really striking. In one recent study, Tim Kaiser, Marco Del Giudice, and Tom Booth analyzed personality data from 31,637 people across a number of English-speaking countries. The size of global sex differences was D = 2.10 (it was D = 2.06 for just the United States). To put this number in context, a D= 2.10 means a classification accuracy of 85%. In other words, their data suggests that the probability that a randomly picked individual will be correctly classified as male or female based on knowledge of their global personality profile is 85% (after correcting for the unreliability of the personality tests).

Consistent with prior research, the researchers found that the following traits are most exaggerated among females when considered separately from the rest of the gestalt: sensitivity, tender-mindedness, warmth, anxiety, appreciation of beauty, and openness to change. For males, the most exaggerated traits were emotional stability, assertiveness/dominance, dutifulness, conservatism, and conformity to social hierarchy and traditional structure.

This basic pattern of findings was replicated in another recent large-scale survey of narrow personality traits conducted on nearly a million people across 50 countries. Using different personality tests, and averaging across all countries, Tim Kaiser found a D = 2.16, which is very similar to the effect size found in the other study on English-speaking countries. While there was cross-cultural variation in the effect, there was a general trend for more developed, individualistic countries with higher food availability, less pathogen prevalence, and higher gender equality to show the largest sex differences in global personality [6].

In particular, Scandinavian countries consistently showed larger-than-average sex differences in global personality, together with the US, Canada, Australia, the UK, and other Northern and Eastern European Countries. The countries with the smallest sex differences in global personality included several Southeast Asian countries. To be sure, there wasn't a perfect correlation between more developed, gender-egalitarian countries and sex differences (e.g., Russia displayed the largest sex difference with D = 2.48). But even Pakistan-- the country with the smallest sex differences in global personality in the world according to this study-- had a D = 1.49. This means that even when you look around the world for the country with the smallest sex difference in global personality, the classification accuracy of that country is still 77%!

These numbers dovetail with a number of studies showing a similar level of classification looking at whole brain data. By applying a multivariate analysis of the whole brain, researchers are now able to classify whether a brain is male or female with 77%-93% accuracy (see here, here, here, here, and here). In fact, some recent studies using the most sophisticated techniques have consistently found greater than 90% accuracy rates looking at whole brain data (see here, here, and here). While this level of prediction is definitely not perfect-- and by no means do those findings justify individual stereotyping or discrimination-- that's really high accuracy as far science goes [7].

All of this data is really hard to ignore and dismiss out of hand.

Posted by at December 14, 2019 8:52 AM