Verified by 
Sometimes sexual diversity researchers will produce a study showing men and women are psychologically different in some way. Not Mars versus Venus different, but different nonetheless. Other researchers might disagree citing a study that finds no psychological sex differences. In an impressive new study, Zell, Krizan, and Teeter (2015) reviewed 100s of past research findings and came to the conclusion that men and women are not very different psychologically. They came to this conclusion using a form of meta-analysis called "metasynthesis."
Meta-analysis is extremely useful for determining if, and by how much, men and women actually differ. Any single research study probably misses the mark at least a little bit in estimating the “true” size of psychological sex differences. Meta-analysis, in contrast, is when researchers simultaneously look across many studies and estimate the overall sex difference quantitatively, often expressed in terms of a “d” metric. A positive d value such as +0.50 typically indicates men are moderately higher on a psychological measure, a negative value like -0.50 indicates women are moderately higher. Below are some d values of varying strengths that have been observed in studies on human sex differences:
A d value of -0.20 has been observed for sex differences in trust (Feingold, 1994). The size of this sex difference is considered “small” and indicates 58% of women are higher than average man in trust (based on Cohen's U3).
A d value of +0.50 has been observed for sex differences in spatial rotation skills (Silverman et al., 2007). The size of this sex difference is considered “moderate” and indicates 69% of men are higher than average woman in spatial rotation skills.
A d value of +0.80 has been observed for sex differences in physical aggression (Archer, 2004). The size of this sex difference is considered “large” and indicates 79% of men are higher than average woman in physical aggression.
A d value of -1.00 has been observed for sex differences in tender-mindedness (Feingold, 1994). The size of this sex difference indicates 84% of women are higher than average man in tender-mindedness.
A d value of +2.00 has been observed for sex differences in throwing distance among children (Thomas & French, 1985). The size of this sex difference indicates 98% of boys throw farther than the average girl.
As I have noted in earlier posts, sex differences with larger d values are not “more real” than smaller-sized sex differences (see here). All men do not have to be taller than all women for a sex difference in average height to be “real” and have important social consequences. Sex differences with larger d values also are not necessarily more attributable to evolution or biology, and smaller sex differences are not more cultural or due to learning than larger sex differences. Meta-analysis alone cannot provide answers to such questions, as Zell, Krizan, and Teeter (2015) rightly note.
The observed d from a meta-analysis, whatever the value, is useful to the degree it represents fairly and systematically collected findings across many different samples, research labs, and time periods. Meta-analysis and observed d statistics give researchers much more confidence in declaring that men and women are, or are not, psychologically different to varying degrees and whether those differences are dependent on particular types of measures, geographic areas, or time periods.
Hyde (2014), for example, reviewed several psychological sex differences and concluded there are relatively moderate to large sex differences in spatial rotation abilities, agreeableness, sensation seeking, interests in things versus people, physical aggression, certain sexual behaviors (e.g., masturbation and pornography use), and attitudes about casual sex. Smaller sex differences exist in measures of gregariousness, reward sensitivity, conscientiousness, negative affectivity, relational aggression, and self-esteem. Some of these sex differences persisted in size across cultures and time periods, others did not (see also, Lippa, 2009; Schmitt, 2014).
In the recent study published in American Psychologist, Zell, Krizan, and Teeter (2015) conducted a “metasynthesis” in which they pulled together 106 previous meta-analyses of psychological sex differences across three areas: Social/personality variables, cognitive measures, and well-being. They concluded that, overall, men and women are not that psychologically different, with an overall d value of 0.21. Three important caveats should be noted about these conclusions.
First, the Zell et al. (2015) study was very limited in scope. They only looked at areas where researchers have actively questioned the existence of sex differences, likely limiting their findings to psychological sex differences so contentiously slight that they have been frequently and repeatedly subjected to meta-analyses. Conclusions about the “real” degree of psychological sex differences should evaluate a much wider range of variables. How wide? Well, it is better in science when one has an organizing theory to heuristically guide how one looks for sex differences. Evolutionary psychologists expect human sex differences to occur only in those domains where ancestral men and women faced different adaptive problems and sexual selection pressures (Okami & Shackelford, 2001).
For instance, evolutionary psychologists expect the sex who has lower levels of obligatory parental investment (in humans, males) to be higher in “sociosexuality” (i.e., willingness to engage in sex without heavy commitment). Human sex differences in sociosexuality have been demonstrated as culturally universal in a study of 48 nations (Schmitt, 2005) and again in a study of 53 nations (Lippa, 2008), with both studies finding the exact same size of worldwide sex differences with men being higher than women, d = +0.74. This is larger than any of the sex differences in the recent study by Zell et al. (2015), but it was not considered and has not been subjected to a “meta-analysis” as it is a largely uncontentious empirical finding (see also here). Sexual diversity associated with sociosexuality is just one example of the dozens of psychological sex differences expected from theories within evolutionary psychology.
Ellis (2011a, 2011b) used his evolutionary neuroandrogenic theory as a guide to examine psychological sex differences and amassed evidence of 65 apparently universal sex differences. These sex differences were shown to be universal across cultures, with not a single replication failure across 10 studies (probably too tough a criterion leading to an under-reporting of actual psychological sex differences). Using evolutionary theory to guide researchers how to look for sex differences versus when to expect no differences (i.e., domains where ancestral men and women have not faced different adaptive problems) yields a very different conclusion from the atheoretical view that men and women are largely indistinguishable overall.
Second, Zell et al. (2015) did not address the very informative cross-cultural variations in the size of many psychological sex differences. Zell et al. reported that the sex differences they did find were "largely constant across age, culture, and time period" (p. 17). However, sex differences in many aspects of personality, sexuality, and cognition are actually much larger in cultures with more egalitarian sex role socialization and greater sociopolitical gender equity. This includes sex differences in extraversion, agreeableness, conscientiousness, neuroticism, openness, Machiavellianism, Narcissism, psychopathy, social dominance orientation, dismissing attachment, intimate partner violence, spatial location ability, spatial rotation ability, crying behavior, depression, benevolence values, love, empathetic occupational preferences, enjoying casual sex, mate preferences for attractiveness, self-esteem, and subjective well-being (Schmitt, 2014). Even sex differences in physical traits such as height, obesity, and blood pressure are conspicuously larger in cultures with more egalitarian sex role socialization and greater sociopolitical gender equity. This suggests it is unlikely that larger psychological sex differences are due to more traditional sex role socialization or patriarchy. Again, evolutionary theories involving life history strategies and ecological factors may be better at explaining the size of psychological sex differences, in this case how and why sizes vary across cultures (see Schmitt, 2014).
Third, Zell et al. (2015) did not utilize informative multivariate approaches that previously have revealed very large psychological sex differences (Del Giudice, 2009; Del Giudice, Booth, & Irwing, 2012). Rather than taking the average sex difference across each psychological dimension on its own, Del Giudice et al.’s multivariate method is to examine all psychological dimensions under consideration simultaneously (controlling for collinear overlap among dimensions). From a multivariate perspective, lots of small ds may be “additive” and create “planetary-size” sex differences when examined together (e.g., Del Giudice et al., 2012, found less than 10% overlap in men's and women's personality traits when looking across 16 dimensions simultaneously). By thinking about sex differences in terms of multidimensional space, this approach is probably a fairer evaluation of whether men and women differ, overall, within a particular multidimensional domain (such as “personality” or “cognition”).
In short, it is probably not true that psychological sex differences should be described as trivially small overall, especially if you know what to look at (heuristically guided by evolutionary theory), where to look (across a wide range of cultures), and how to look (using multivariate approaches). Men and women are members of the same species, but psychologically there are important differences that should not be overlooked if we are to maximize everyone’s medical, mental, and sexual health.
Archer, J. (2004). Sex differences in aggression in real-world settings: A meta-analytic review. Review of General Psychology, 8, 291-322.
Del Giudice, M. (2009). On the real magnitude of psychological sex differences. Evolutionary Psychology, 7, 264-279.
Del Giudice, M., Booth, T., and Irwing, P. (2012). The distance between Mars and Venus: Measuring global sex differences in personality. PLoS ONE, 7, e29265.
Ellis, L. (2011a). Identifying and explaining apparent universal sex differences in cognition and behavior. Personality and Individual Differences, 51, 552-561.
Ellis, L. (2011b). Evolutionary neuroandrogenic theory and universal gender differences in cognition and behavior. Sex Roles, 64, 707-722.
Feingold, A. (1994). Gender differences in personality: A meta-analysis. Psychological Bulletin, 116, 429-456.
Hyde, J. S. (2014). Gender similarities and differences. Annual Review of Psychology, 65, 373-398.
Lippa, R. A. (2009). Sex differences in sex drive, sociosexuality, and height across 53 nations: Testing evolutionary and social structural theories. Archives of Sexual Behavior, 38, 631-651.
Okami, P., & Shackelford, T. K. (2001). Human sex differences in sexual psychology and behavior. Annual Review of Sex Research, 12, 186-241.
Schmitt, D. P. (2005). Sociosexuality from Argentina to Zimbabwe: A 48-nation study of sex, culture, and strategies of human mating. Behavioral and Brain Sciences, 28, 247-275.
Schmitt, D.P. (2014). The evolution of culturally-variable sex differences: Men and women are not always different, but when they are…it appears not to result from patriarchy or sex role socialization. In Weekes-Shackelford, V.A., & Shackelford, T.K. (Eds.), The evolution of sexuality (pp. 221-256). New York: Springer.
Schmitt, D. P., Realo, A., Voracek, M., & Allik, J. (2008). Why can't a man be more like a woman? Sex differences in Big Five personality traits across 55 cultures. Journal of Personality and Social Psychology, 94, 168-182.
Silverman, I., Choi, J., & Peters, M. (2007). The hunter-gatherer theory of sex differences in spatial abilities: Data from 40 countries. Archives of Sexual Behavior, 36, 261-268.
Thomas, J. R., & French, K. E. (1985). Gender differences across age in motor performance: A meta-analysis. Psychological Bulletin, 98, 260-282.
Zell, E., Krizan, Z., & Teeter, S. R. (2015). Evaluating gender similarities and differences using metasynthesis. American Psychologist, 70, 10-20.
