Psychology: More alike than different

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Two books debunk gender differences in the brain, discovers Virginia Valian.

Delusions of Gender: How Our Minds, Society, and Neurosexism Create Difference/The Real Science Behind Sex Differences

W. W. Norton/Icon Books: 2010. 338 pp./368 pp. $25.95/£14.99 9780393068382 | ISBN: 978-0-3930-6838-2

Trying to distinguish the female from the male brain is a trap that many writers fall into. Two books provide a welcome corrective by reviewing scientific evidence showing that the sexes are more alike than different, and that small sex differences are not fixed but change with context and across generations.

Cordelia Fine's Delusions of Gender is aimed at a broad audience and debunks the egregious exaggerations common in popular books. Rebecca Jordan-Young's Brain Storm is directed at specialists who are interested in historical studies of the brain, sex hormones and gender differences. Fine, a psychologist at Macquarie University in Sydney, Australia, and Jordan-Young, a medical sociologist at Barnard College in New York, point out the methodological and theoretical limitations of such studies, and demonstrate the part that folk theories about gender differences have played in casting the sexes as inherently dichotomous.

Both books note that correlation and causation are often confused when looking at the results of brain or hormonal differences on behaviour — 'causing', for example, boys to be better than girls at mental rotation of three-dimensional objects. Too many studies ignore mediating variables and alternative explanations and exaggerate the extent of gender differences. Both accounts direct attention to the full range of results, including those that researchers might otherwise ignore.

Credit: H. PYLE/CORBIS

Although males and females differ in many ways, the authors note that the same could be said of any two groups — people with different hair colour or degrees of economic security, say. Because men and women have distinct reproductive functions, controlled in part by different hormones, we are prone to interpret their behavioural and psychological differences as not only biologically mediated but mandated. Putative brain differences are seized on as sex-based and other hypotheses can be overlooked.

To illustrate, Fine considers the corpus callosum linking the two brain hemispheres. The fact that it seems to be thicker in women than in men is sometimes associated with women's supposed greater verbal skill. According to this 'just-so' reasoning, a thicker corpus callosum may allow easier integration of information from the two sides of the brain. But, as both authors point out, the reported differences in corpus callosum size are unreliable between studies, and seem to be related to brain size. In both sexes, larger brains have a larger callosum, but in smaller brains they make up a greater percentage of brain volume. So if a sex difference in the corpus callosum exists, it is likely to be a by-product of the basic difference in average brain size — a fact that is missed by a focus on sex. Unless researchers conduct analyses that divide groups in ways other than gender, the real story might be missed.

The behaviours of both sexes are remarkably similar despite their presumed brain and hormonal differences. Although men's brains are roughly 10% larger than women's, the IQs of both genders are roughly equal, and most experiments in cognitive processing in children and adults reveal no sex differences.

Brain Storm: The Flaws in the Science of Sex Differences

Harvard University Press: 2010. 408 pp. $35 9780674057302 | ISBN: 978-0-6740-5730-2

In deflating spurious claims about the influences on sex differences, the books risk underestimating those effects. Consider congenital adrenal hyperplasia (CAH), a major topic in Jordan-Young's book, in which the fetus is exposed to higher levels of androgens than are unaffected children. This causes emotional and physical symptoms ranging in girls from early-onset puberty to infertility. Jordan-Young concludes that children with and without CAH are more alike than different. But in my view, she underestimates the differences between girls with CAH and their unaffected peers in two domains.

Although the data are mixed, girls with CAH seem to be better at mentally rotating shapes than are unaffected girls. The results are clearer for toy preferences: when asked to choose from various 'masculine', 'feminine' and 'neutral' toys, girls with CAH show more interest in masculine and less interest in feminine toys than girls who are unaffected. Boys with and without CAH are indistinguishable in their choices.

Whether a toy is perceived as masculine or feminine, however, depends on who plays with it; as Jordan-Young notes, children's play preferences have changed over time. Some activities once seen as masculine, such as playing with construction toys and balls, are now seen as neutral and on a par with books and puzzles. Dolls, by contrast, have remained feminine. Toy-preference studies show that girls divide their time equally among masculine, feminine and neutral toys, whereas boys tend to eschew feminine ones. So the girls with CAH are acting like control boys. But we cannot tell whether that is because they prefer masculine toys or are taking less interest in feminine ones. The way such questions are posed affects where one searches for answers.

So why do boys avoid dolls? They learn early that they will pay a social price for showing 'deviant' feminine interests. Girls can like the same toys as boys with little fear of negative social consequences, as demonstrated by the different connotations of 'sissy' and 'tomboy'. For girls with CAH, there is another social issue: many undergo genital surgery in infancy and have impaired fertility. Girls' knowledge about their condition may trigger concerns about their femininity and lead to an avoidance of toys such as dolls.

To some extent, both books are brooms that sweep too clean. I contend that one cannot dismiss sex differences in mental rotation of three-dimensional objects as ill-documented or easily reversed, as both authors do. The difference is robust and difficult to neutralize; its precursors are seen in infancy. Laying such distinctions only at the doorstep of social psychology — in different expectations of the two sexes, for example — closes off inquiry into underlying cognitive processes. Similarly, there is almost nothing about sex differences in mathematics and science in Jordan-Young's book, and only a cursory treatment in Fine's.

I share both authors' conclusions that the jury is still out over whether hormonal and neural sex differences are linked to behavioural divergence. I also agree that social context is often a more likely source of sex differences. Most behavioural sex differences are small, and there is no direct mapping between a behaviour and a set of neural mechanisms. Different neural mechanisms can be responsible for the same behaviour.

It is no bad thing that there are many hypotheses still to test. Eventually, unsupported preconceptions about what counts as masculine and feminine will be discarded. Until then, people such as Fine and Jordan-Young are just who we need.

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Valian, V. Psychology: More alike than different. Nature 470, 332–333 (2011). https://doi.org/10.1038/470332a

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