Recent publication in PLoS ONE by psychologist Del Giudici and colleagues [i] has reignited the debate about just how "naturally" different men and women are. Del Giudici et al. state that their findings of a “pattern of global sex differences…may help elucidate the meaning and generality of the broad dimension of individual differences known as "masculinity-femininity".”

In a commentary, psychologist Dario Maestripieri [ii] gushes that this study has finally demonstrated that “when it comes to personality men and women belong to two different species.” In spite of the hoopla and pronouncements that men are indeed from Mars and women for Venus this study, and the commentaries, ignore that trying to assess and explain similarities and differences between human genders and sexes is very complicated and quite messy. Apparently, it also makes people act a little silly.

There are three major problems with the conclusions being drawn from study: a) “gender” and “sex” are used interchangeably, b) evolved differences in men and women are not being measured, and c) relevant biological and anthropological datasets are ignored. Let me just review these problems and leave you with a plea for a bit of sanity and some scientific integrity when it comes to thinking and talking about men and women.

“Sex” and “Gender” are not the same thing. Sex is a biological state that is measure via chromosomal content and a variety of physiological and developmental measures. Gender is the roles, expectations and perceptions that a given society has for the sexes. Most societies have two genders on a masculinity-femininity continuum, some have more. The two are interconnected, but not the same thing. We are born with a sex, but acquire gender and there is great inter-individual diversity within societies and sexes in regards to how sex and gender play out in behavior and personality. There is an extensive body of literature demonstrating this, but many researchers interested only in definitive distinctions between men and women choose to disregard it.

To measure evolutionary differences in behavior within a species is extremely difficult, but there are at least two basic methodological approaches that are required. First, assessments must be comparative across more than one population of the species of interest. Second the traits being measured must have some way of being linked or connected with heritable aspects of human physiology or behavior that has an effect on overall fitness, and they must be assessed via measures that are accessible, and replicable, across different populations in the species. Del Giudici et al. used a large questionnaire sample of mostly white, educated Americans. Relative to the global diversity in cultural structure, this is a limited sample and not a comparative evolutionary one for the species.

Their data come from assessments of 15 personality variables using scales such as “reserved vs. warm,” “serious vs lively,” “tolerates disorder vs. perfectionistic,” and “shy vs socially bold.” These are indeed personality assessments but they are mired in cultural contexts and meanings, not easily transferable across human societies in time and space, and extremely difficult, if not impossible, to connect, quantitatively, to any aspect of human physiology, neurology, or other structured, identifiable, target for natural selection to act on. Also, these are most likely not static traits of individuals, but rather dynamic states that are fluid over the lifetime.

Finally, when talking about evolved differences in behavior between males and females one cannot make statements like “when it comes to personality men and women belong to two different species” without noting the biological reality that we are, indeed, the same species. There are no consistent brain differences between the sexes [iii], there is incredible overlap in our physiological function [iv], we engage in sexual activity in more or less the same patterns [v], and we overlap extensively in most other behavior as well. There are some interesting re-occurring differences, particularly in patterns of aggression and certain physiological correlates of reproduction, muscle density, and body size. However, anthropological datasets show enormous complexity in how and why men and women behave the ways that they do [vi]. Studies in human biology and anthropology regularly demonstrate a dynamic flexibility and complex biocultural context for all human behavior, and this is especially true for gender.

Del Giudici et al. and Maestripieri are trying to counter Janet Shibley-Hyde’s “gender similarities hypothesis” [vii] because they “know” that men and women are more different than similar. There are many valid points of contention in regards to Shibley-Hyde’s seminal paper and Del Giudici et al. bring up an important methodological one, but do not provide an actual assessment and analysis of the overall data set and meta-analyses that Shibley-Hyde used [viii]. My concern is not so much with some good back and forth in the peer reviewed literature, rather it is with the blogospheres’ and the public’s response to the article and to yet another flare-up in over simplistic assertions about the way that men and women “are” by nature.

There is something about avidly trying to prove men and women are different, or the same, that makes people lose their mind a bit. No matter how much some want it to be true, it is just not that simple; there are no clear cut and easy answers to why we do what we do, and why men and women sometimes have problems getting along. To ignore the enormous wealth of data on how men and women are similar AND different and to try to tackle this enormously complex reality via one-dimensional approaches is just poor science.

[i] Del Giudice, M., Booth, T., and Irwing, P. (2012). The distance between Mars and Venus: Measuring global sex differences in personality. PLoS ONE 7(1): e29265

[iii] Eliot, L.(2009) Pink brain Blue brain. Houhgton Mifflin Harcourt., Wood, J.L., Heitmiller, D., Andreasen, N.C., Nopoulos, P. (2008). Morphology of the ventral frontal cortex: relationship to femininity and social cognition. Cerebral Cortex, 18, 534–40., Bishop, K. and Wahlsten, D. (1997) Sex Differences in the Human Corpus Callosum: Myth or Reality? Neuroscience and Biobehavioral Reviews 21(5):581-601

[iv] Anne Fausto-Sterling (2000) Sexing the Body: gender politics and the construction of sexuality Basic Books, P.T. Ellison and P.B. Gray Eds.(2009) The endocrinology of social relationships. Harvard University Press Pp. 270-293

[v][v] Herbenick, D., Reece, M., Schick, V., Sanders, S.A., Dodge, B., Fortenberry, J.D. (2010) Sexual behavior in the united states: results form a national probability sample of men and women ages 14-94. J. Sex Med. 7(suppl. 5):255-265

[vi] Nanda, S. (2000) Gender diversity: cross-cultural variations Waveland Press, Donnan, H. and Magowan, F. (2010) The Anthropology of Sex Berg Publishers

[vii] Hyde JS (2005) The gender similarities hypothesis. Am Psychol 60: 581–592.