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Unexpected sex differences in brain development

Sex differences in the brain - weirder than you thought

A team of researchers from the NIH recently released some of the most comprehensive research yet on sex differences in brain development, as measured by high-resolution MRI scans. They followed a large cohort of kids from age 9 through age 22 (this research actually began back in 1995). So the same kids came in to the lab on multiple occasions to have their brain scanned. That method allows you to distinguish individual differences in brain development from sex differences in brain development.

The group found dramatic and significant sex differences in the structure of the cerebral cortex, which by itself isn't that surprising. Other groups have reported similar differences for more than a decade now. What's particularly interesting about this new report is that the NIH group found that sex differences diminish as a function of age, from age 9 through age 22.

To put it another way: after the onset of puberty - when girls start making lots of estradiol and other ‘female' hormones, while boys start making lots of testosterone and other ‘male' hormones - sex differences in the brain actually decrease. The brains of 9-year-old girls and boys are remarkably different - but they grow more and more alike throughout adolescence and into young adulthood.

You can read an abstract of the article, which links to the full text, here. But the NIH authors have posted an online video, available without charge, which dramatically illustrates their point. (Technical notes: this video is designed for Quicktime; it may not run in Windows Media Player. So you may need to adjust your browser settings in order to watch it. If you have trouble seeing the play/pause control, set your "View" option to "Full Screen.") The video begins with some areas in blue - those are the areas which are more mature in the 9-year-old boy - and some areas in pink - the areas which are more mature in the 9-year-old girl. You then watch how the brain develops. Between age 9 and age 22, both the pink areas and the blue areas yield to ‘white' areas - those brain areas which don't show significant sex differences. At age 9, there is hardly any brain area at all which is white. By age 22, more than half the brain - including almost all the frontal cortex - is white, i.e. no significant sex differences.

This finding is counter-intuitive, even to people who think they know something about sex differences and the brain. In the past two years, three semi-scholarly books have appeared, purporting to show that sex differences are primarily socially constructed. Those three books are Delusions of Gender by Cordelia Fine; Brainstorm by Rebecca Jordan-Young; and Pink Brain, Blue Brain by Lise Eliot. These authors all claim that sex differences at birth are negligible, but that our sexist society teaches girls and boys to be different. If that were the whole story, then we ought to find that sex differences in the brain increase as a function of age, particularly after the onset of puberty, when the whole process which Professor Diane Ruble calls "gender intensification" kicks into gear. But the NIH study suggests that the opposite is the case.

All three books of these books have received glowing reviews in the popular press, such as this breathlessly enthusiastic article in Newsweek by the otherwise-usually-reliable Sharon Begley. Yet the recent article by the NIH team, published in the prestigious Proceedings of the National Academy of Sciences (PNAS), has received no mention of any sort in any media. Not one squib. How come? Is it possible that the reporters don't know what to make of this research? Or maybe it's not politically correct?

In fact, the basic finding of the NIH researchers - that sex differences diminish as a function of age - will not come as any surprise to anyone who has worked with children and adolescents, or to anyone who read my second book Boys Adrift. Here's one way to illustrate the point: How long can you sit still, be quiet, and pay attention? If you compare mature adults - say, a 40-year-old woman with a 40-year-old man - you won't find any difference. A average 40-year-old man can sit still, be quiet and pay attention for roughly as long as the average 40-year-old woman.

But now compare a 6-year-old boy with a 6-year-old girl. You will typically find that the attention span of the average 6-year-old boy is shorter than the average 6-year-old girl, especially if he has to sit still and be quiet. In my visits to more than 300 schools over the past ten years, I have found many 6-year-old boys who absolutely have to bounce up and down and make buzzing noises, in order to pay attention.  As one teacher said to me, referring to one young boy in her class:  "When that boy sits down, his brain shuts off."

It's unusual to find a 40-year-old man who absolutely has to bounce up and down and make buzzing noises in order to pay attention.

Sex differences are unexpectedly large in childhood, and diminish as a function of age. Don't you think that's something teachers, and parents, need to know?

Leonard Sax MD PhD is a psychologist, physician, and author of Boys Adrift and Girls on the Edge.