As a basic scientist attempting to understand the mechanistic basis of sex differences in the brain, I found Ben Barres's Commentary article1 of particular interest.

The existence of robust and reliable sex differences in brain regions relevant to the control of reproduction cannot be refuted. However, my research group spent a year of frustration trying to apply the same principles to putative sex differences in cognition. This ended with the epiphany that even the standard laboratory rat shows few, if any, sex differences in the morphometry of regions relevant to cognition such as the hippocampus, and that the learning ability of both sexes is essentially the same2.

A search of the literature revealed that one of the first reports of sex differences or hormonal modulation of learning in rats, published in 1926, cites the 'fact' that such differences are reliably established in humans as supporting evidence (and note, the author was a woman)3. Thus, even the fundamental science of learning in animal models was tainted by bias from its inception. Only with the influx of a large number of women scientists is the notion of superior male spatial ability beginning to be challenged. Data are now being reinterpreted as showing a difference in learning strategy rather than ability4,5.

When I had the fortune to meet Ben Barres a few years ago, he told me his anecdote about losing the ability to cry easily after changing his gender to male, and suggested that sex differences in the lachrymal gland would be an excellent topic for study. At the time, I laughed at his naive notion that a behavioural sex difference would be the result of a peripheral gland. But now, I am not so sure.

Readers are encouraged to add their comments to the Ben Barres Commentary on the Nature News Blog at: does_gender_matter.html.