The New Science of Sex Difference

While scientists across the academy have abandoned the nature ⁄nurture dichotomy, evidence for the influences of society on our biology is greater than ever. This article reviews new developments in the biological sciences – in the sub-fields of genetics, hormones, and neuroscience – with special attention to the implications for sociologists interested in gender. The article closes with an argument that embracing these developments has both theoretical and methodological promise and can enhance rather than harm research and activism regarding gender equality and other social hierarchies. ... any living cell carries with it the experience of a billion years of experimentation by its ancestors. You cannot expect to explain so wise an old bird in a few simple words. – Max Delbrück (1949a,b) In the early 1800s a French biologist named Jean-Baptiste Lamarck, working on the question of what would someday be called ‘‘evolution,’’ proposed that individual animals could pass on acquired as well as inherited traits to their offspring (Bowler 2003). That is, adjustments an organism made to its environment during its life could somehow appear in the biological building blocks of the next generation. His most famous example involved the neck of the giraffe, a feature that bewildered early scientists. Lamarck theorized that each generation of giraffes stretched their neck to reach higher and higher leaves, passing on a slightly longer neck than they had inherited themselves. Likewise, Lamarck speculated, the traits that humans developed over the course of their lives could be inherited by their children. For example, if a man became strong, his children would be born with a greater predisposition for large muscles; if a woman became educated, she would pass onto her children heightened intellectual potential. After Darwin, this model of evolution fell out of favor. In its place was the theory of natural selection: evolution works not through organisms actively responding to the environment, but through random genetic variation and the failure of the maladapted to reproduce. The idea that we could change our genes during our lives and pass on a different genome than the one we inherited came to seem laughably naı̈ve. Emerging research now suggests that Lamarck was onto something. Indeed, our understanding of biology and its relationship to the phenomena of interest to sociologists – cultural ideas, social interaction, and social structures – is undergoing a paradigmatic change (Silverman 2004; Strohman 1997). In this essay I review three biological bases of sex difference and similarity – genes, hormones, and brains – and explore the new research that shows how each mechanism interacts with the socio-cultural context. I conclude by joining the call to reorient our relationship to the life sciences (e.g., Bearman 2008; Franks 2010; Freese et al. 2003; Mazur 2005; Udry 1995). These developments should inspire us to further develop research programs that take advantage of the interaction of biology Sociology Compass 7/4 (2013): 278–293, 10.1111/soc4.12028 a 2013 Blackwell Publishing Ltd and society. Engaging with the biological sciences in this way need not naturalize inequality, though this is an outcome against which we must be vigilant, but rather can offer social scientists stronger tools with which to identify, criticize, and eliminate mechanisms of oppression. How sexually dimorphic are humans? Perhaps the most important thing to understand when approaching contemporary research on sex differences and similarities is that men and women are overwhelmingly alike. When we consider the full range of biological adaptations to sexual reproduction, humans are not particularly sexually dimorphic. Some species show dramatic differences between males and females in appearance; we do not. Moreover, because we are not particularly dimorphic in appearance, we should expect significant overlap in our abilities and interests, considering that morphological sexual dimorphism correlates with divisions of labor. In fact, meta-analyses aimed at summarizing the literature on human sex differences and similarities in traits, personality, cognitive abilities, sexuality, temperament, and motor skills offer better evidence for similarity than difference, even in the face of cultural and social structural forces that reflect a gender binary (Else-Quest et al. 2006; Hyde 2005; Petersen and Hyde 2010; Wallentin 2009). On 30 percent of variables scientists have found no compelling evidence gender difference; on an additional 48 percent of variables, scientists have documented a small difference (one for which 54–64 percent of one sex scores better than 50 percent of the other). Together, these included reading comprehension and abstract reasoning; talkativeness, likelihood of self-disclosing to friends and strangers, tendency to interrupt others, and assertiveness of speech; willingness to help others, negotiation style, approach to leadership, and degree of impulsiveness; self-esteem, symptoms of depression, coping strategies, life satisfaction and happiness; vertical jumping ability, overall activity levels, balance, flexibility; willingness to delay gratification and attitudes about cheating; likelihood of wanting a career that makes money, offers security, is challenging, and brings prestige; and some measures of sexual attitudes and experiences (e.g., disapproval of extramarital sex, levels of sexual arousal, and sexual satisfaction). Scientists document medium-sized gender differences (one for which 65–74 percent of one sex scores better than 50 percent of the other) on 15 percent of variables and large or extra large differences (where at least 75 percent of one sex scores better than 50 percent of the other) on the remaining eight percent. The largest gender differences were for some measures of physical ability, especially throwing, and some measures of sexuality, including masturbation incidence and likelihood of approving of casual sex. In addition, two traits show very strong sexual dimorphism: sexual identity (most men identify as male and most women identify as female) and sexual object choice (most men are sexually interested in women and most women in men) (Hines 2009). Rebecca Jordan-Young (2010) argues that these data establishing sex differences and similarities should be thought of not as evidence of an unchanging reality, but as a mere snapshot of what is really a moving target. Indeed, most of these differences and similarities grow or shrink as we look across time, across cultures, or within subcultures in a given country (Else-Quest et al. 2010; Wood and Eagly 2012). Likewise, the results of many tests can be easily manipulated in the laboratory, revealing that context, framing, priming, instruction, practice, and other mechanisms all influence subjects’ performances (Cherney 2008; Fine 2010). Nevertheless, we are a species that reproduces sexually and, so, there are biological differences between men and women. In this paper, I review three areas of inquiry in The New Science of Sex Difference 279 a 2013 Blackwell Publishing Ltd Sociology Compass 7/4 (2013): 278–293, 10.1111/soc4.12028 which scientists have documented clear biological differences between men and women: genetics, hormones, and brain structure and function. In each case I review the differences that have been established through replicated research on humans and then offer an overview of our emerging understanding of how these biological processes interact with the socio-cultural environment.