Highlights: February 3–9

Here's what we loved reading this week, including: the health benefits of having friends, the art of personal choice, and the bugs in your brain.

When Is a Sex Difference Real?

How much of a difference is needed before a sex difference is real?

According to a recent study in the prestigious Journal of Personality and Social Psychology, men and women are not all that different when it comes to their sexuality (Carothers & Reis, 2013). No dichotomous Mars versus Venus thinking here, thank you very much! Indeed, sexual stereotyping can have a potent corrupting influence on our lives. We all suffer when solely characterized as nothing more than our gender, instead of unique individuals with complex sexual personalities. When people hold highly gendered beliefs about men and women, and when these beliefs are at odds with empirical reality, it is the responsibility of research psychologists to inform the public about what we know that is not so, and what evidence we have to dispel the myths of gender. Such work is great sexual science. Unfortunately, the study by Carothers and Reis makes some big mistakes, both conceptually and empirically. Not so great. Not so science. Here is why.

Conceptually, it is ironic that a paper rooted in concerns about dichotomous thinking about sex differences—Mars versus Venus labeling—sets out to portray sex differences in a dichotomous manner, as either categorical or dimensional. Sex differences must be understood dichotomously, but men and women must not be? Such irony would be funny, if not for it also serving as a critical conceptual shortcoming. Psychological attributes are almost always multiply determined (even if influenced by genetic and hormonal factors related to biological sex). To imply that the sexes do not really differ on an attribute if they do not nearly entirely differ on that attribute is scientific nonsense (see an informative post by Michael Mills here). Life is more complicated than that, and Carothers and Reis do partially recognize this.

For instance, they note in their study that whether an attribute is dimensional or categorical, this does not tell us definitively about the origins of the sex difference (e.g., whether it is socially or biologically caused; or both in complex ways): “Our findings are silent with respect to the question of whether gender differences in the variables we studied are caused primarily by biological factors or experience (Eagly & Wood, 1999). In our view, both biological and social causes are essentially continuous, leading individuals to develop various proclivities and dispositions to one or another extent, and encouraging them to follow certain developmental pathways to a greater extent than others (Archer, 1984; Halpern, 2012; Maccoby, 2002). It is unlikely that any of these pathways are fully discrete.” Well said. I couldn't agree more.

Are the Sexes From Different Categories?

So, what did the authors really find when it comes to sexuality? In evaluating pre-existing datasets, the authors combined multiple items (specific questions on sex surveys) and evaluated whether a dichotomous structure is formed by the differing responses of men and women. That is, they focused on whether men’s and women’s responses place them into two distinct sexual groups. More precisely, the researchers tested to see if when men scored higher than women on one sex question (e.g., willingness to have sex without love), were men also higher on than women on all other theoretically-similar sex questions? And did the responses cluster together so closely among men and so differently among women as to form a “taxon” (or a dichotomous category dependent mostly on biological sex)?

Empirically, this seems like a reasonable question. As addressed in the study, though, it is not. A huge problem with the study is, in most cases, the combined sexuality questions (questions the authors argued should form clusters dichotomously distinguished by biological sex) really should not be expected to cluster together as a taxon, especially from the theoretical perspective of evolutionary psychology.

Take their archival analysis of the National Health and Social Life Survey dataset (this is a large nationally representative sample of the USA in which some sexuality questions were asked). Some of the questions—such as the appeal of sex with more than one partner, the appeal of having sex with a stranger, and the willingness to have sex without love—should be reasonably expected to co-vary together. On their surface, all the items seem relevant to what evolutionary psychologists call “short-term mating psychology” or “unrestricted sociosexuality.” Even so, from an evolutionary perspective, these items should not co-vary entirely by sex.

As I have posted over and over again in this blog, evolutionary psychologists expect that only some men pursue short-term mating strategies (e.g., given their own mate value, physical attractiveness, attachment experiences, local pathogen levels, local sex ratios, and so forth; see Gangestad & Simpson, 2000). Not all men are short-term maters at all times. If this is the view that Carothers and Reis have of evolutionary psychology, it is an inaccurate one. And some women are also expected to strategically pursue short-term mating (albeit for different underlying functional reasons than men do). Thus, the Carothers and Reis theoretical expectation that all men are short-term maters and all women are not—and so we should expect taxons to form on questions about short-term mating psychology—is wrong-headed at its core. See also my previous posts on this here and here.

Sadly, it gets worse. Carothers and Reis strangely included in their evaluation of this particular taxon many other items—such as reliably having orgasms and frequency of masturbation—that represent very different types sexual psychology. Including these questions alongside questions about short-term sexuality is a very odd decision from an evolutionary theory perspective. Seriously, why include these questions?

Most theories within the paradigm of evolutionary psychology expect psychological adaptations to be largely modular, turning on and off depending on factors such as sex, developmental experience, and local ecological condition (among other reasons). Combining radically diverse kinds of sexuality questions is an exceptionally poor empirical test of whether evolved sexual psychologies are taxonic by sex. Thus, the combinations of questions used by Carothers and Reis to evaluate the taxonic nature of sex differences in sexuality were problematic conceptually and empirically. Not so great. Not so science.

A Finding that Supports Evolutionary Psychology

Actually, one very interesting finding in the review by Carothers and Reis was that explicit measures of sociosexuality (examined across several studies) did take a somewhat taxonic form, but not by sex. Carothers and Reis found about 50% of men and a few women formed an unrestricted (or permissive) sociosexual taxon, whereas about 50% of men and almost all women formed a restricted sociosexual taxon. As Carothers and Reis noted, “As in Run 1, the Bayesian probabilities distinguished two groups, with approximately half the men and a few women in the high-SOI taxon, and most of the women and nearly half the men sorted into the low-SOI complement.”

These findings are largely what would be expected from an evolutionary perspective on sociosexuality. When combining sociosexuality findings with mate preference findings, Carothers and Reis further noted: "the Bayesian membership probabilities tended to sort men and women into separate groups with fair accuracy in the expected directions—the high end of sociosexuality and low end of mate selectivity were more likely to contain men, and the opposite ends more likely to contain women." Bingo! Welcome to evolutionary psychology.

When More Comparisons are Better Comparisons

One last point. Carothers and Reis should be commended for trying to evaluate sex differences across a lot of questions at the same time, what is called taking a multivariate perspective. Often, researchers look only at one part of our psychological make-up, and make conclusions about sex differences based on numerous individual comparisons. Looking at sex differences this way can be misleading, such as with the so-called gender similarities hypothesis (Hyde, 2005).

In a multivariate study by Del Giudice et al. (2012), men’s and women’s personalities were contrasted along 16 fundamental personality dimensions at the same time. By looking at all these dimensions simultaneously, researchers were able to control for the fact that many psychological dimensions overlap with one another. Because of this overlap, scientists cannot just add up all the differences between men and women, or just average across the differences for each psychological dimension. Instead, the most informative approach for evaluating the overall sex difference is to examine all the sex differences across multiple dimensions at the same time (whilst also controlling for all overlap among dimensions). It can be a bit tricky, but when they did this in the statistically appropriate manner, Del Giudice et al. found the sexes differed enormously, with only a 10% overlap in their overall personality distributions. Not Mars versus Venus exactly, but certainly not extreme gender similarity.

Carothers and Reis may have been well-intentioned, but many aspects of their study were conceptually weak and the conclusion of many media reports that men and women do not differ in their sexuality is empirically very wrong. Sexual science deserves better. Men and women show marked differences in many sexual attributes, particularly in sociosexual attitudes, short-term mating tendencies, and expressed mate preferences. This is some of what we know about important sex differences in sexual psychology, and a little bit about how we know it. To pretend otherwise is not only bad science, it can be risky for the sexual health of men and women everywhere.


Carothers, B.J., & Reis, H.T. (2013). Men and women are from Earth: Examining the latent structure of gender. Journal of Personality and Social Psychology, 104, 385-407.

Del Giudice M., Booth T., Irwing, P. (2012). The distance between Mars and Venus: Measuring global sex differences in personality. PLoS ONE, 7, e29265. doi:10.1371/journal.pone.0029265.

Gangestad, S.W., & Simpson, J.A. (2000). The evolution of human mating: Trade-offs and strategic pluralism. Behavioral and Brain Sciences, 23, 573-587.

Hyde, J.S. (2005). The gender similarities hypothesis. American Psychologist, 60, 581–592.