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A spirited polemic takes aim at biological sex differences but misses opportunities to highlight relevant science

Testosterone Rex: Myths of Sex, Science, and Society

Cordelia Fine
266 pp.
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How and why do the sexes differ? And why do we care? Few questions generate as much controversy and debate in both scientific and public arenas.

In her book Testosterone Rex: Myths of Sex, Science, and Society, Cordelia Fine tackles the question from the perspective that has generated the most discussion: biological contributions to sex differences. Her goal is to show that sex inequalities in society cannot be attributed to evolved sex differences in the brain and to men’s high levels of testosterone.

A focus of Fine’s book (and her other writings) concerns brain sex differences. She cogently reveals, for example, that the differences are relatively small (and so do not support the notion that men and women have qualitatively different brains) and have rarely been directly linked to behavior. Brain size, she notes, is not the sole, or even main, indicator of function (women function well with hearts that are smaller than those of men), and sex differences in the brain may simply reflect alternative paths to the same outcome (again, consider the heart).


Behavioral sex differences may have biological underpinnings, but they are not immutable.

Hormonal differences across men and women are generally considered to contribute to psychological sex differences in two primary ways: During sensitive periods of development, they produce long-lasting changes to brain structure and behavior (“organizational” effects), and later in life, they produce temporary alterations to the brain and behavior as hormones circulate in the body (“activational” effects). Fine focuses on activational effects, that is, links between behavior and circulating hormones (primarily testosterone), which are known to be context-dependent and bidirectional. She does not discuss methodologically rigorous studies of organizational effects, which have shown that exposure to androgens during prenatal development has clear effects on sex-related behavior, including interests and abilities (1). She thus fails to provide a balanced presentation of the role of hormones in sex-related behavior.

I welcome and applaud Fine’s efforts to ground policy in science and to spotlight the false reasoning and dichotomies that appear in popular books and some policies (such as single-sex education). I also recognize (and regret) the long history—and present—of using biology to justify inferior treatment of women. This no doubt contributes to resistance to evidence of biological differences among those seeking gender equality.

The challenge is not to dismiss biological explanations of sex differences but to articulate clearly their implications. We can accept that biology contributes to behavioral sex differences and simultaneously argue that gender inequalities are not intractable. Rather than rejecting biological differences, we must seek to reveal the nonsense in the arguments that brain and behavioral sex differences justify discrimination, segregation, and differential treatment of the sexes.

It is also important to emphasize the fact that biologically influenced behavioral sex differences are not determined or immutable. Most of us make this distinction with respect to other phenotypes; for example, we exercise and eat well to change our genetically influenced appearance and risk of disease. Behaviors with a strong biological influence may still be modified by environmental factors. Conversely, environmental causation does not imply easy modification. The persistence of racism and sexism illustrates the difficulty of countering social forces.

In considering behavioral change, however, we need to recognize that interventions to reduce sex differences are generally unidirectional. Most efforts to increase gender equity focus on making girls and women more like boys and men (for example, improving girls’ math and spatial skills), rather than making boys and men more like girls and women (for example, improving boys’ emotion recognition skills). This makes sense in some ways (for example, increasing women’s participation in STEM careers fulfills societal needs for scientists and engineers), but it does not make sense in others (for example, failing to increase men’s participation in social service careers hinders efforts to increase the number of child and health care workers). The differential value placed on male-typical behaviors is evidence for the misogyny that concerns Fine and should concern us all.

Fine has done us a service by reminding us of the dangers of misapplications of research to policy and of the draw of biological explanations, even when they do not fit. But there are also dangers in ignoring good science. Actions to achieve gender equality do not rest on discarding the evidence for biological influences on sex differences.


  1. S. A. Berenbaum, A. M. Beltz, Curr. Opin. Behav. Sci. 7, 53 (2016).

About the author

The reviewer is at the Departments of Psychology and Pediatrics, The Pennsylvania State University, University Park, PA 16802, USA.