The most profound difference between girls and boys is not in any brain structure per se, but rather in the sequence of development of the various brain regions. The different regions of the brain develop in a different sequence, and different tempo, in girls compared with boys -- this is the key insight from the past five years of neuroscience research in brain development. The world's largest study of brain development in children, conducted primarily by researchers at the National Institutes of Health (NIH), has demonstrated dramatic differences in the trajectories of brain development in girls compared with boys. You can download the full text of the study by clicking here.

The NIH/NIMH study, entitled "Sexual dimorphism of brain developmental trajectories during childhood and adolescence," is reprinted by permission of Elsevier from the journal NeuroImage, volume 36, number 4, pages 1065-1073, July 15 2007.

In the figure below, from the NIH study, shows trajectories of brain development in girls (red line) and boys (blue line), with 95% confidence intervals above and below each line.  The arrow indicates the “inflection point,” roughly the halfway point in brain development. Girls reach the inflection point just before age 11 years; boys do not reach the inflection point until just before age 15 years. A young woman reaches full maturity, in terms of brain development, between 21 and 22 years of age. A young man does not reach full maturity, in terms of brain development, until nearly 30 years of age.

Boys and Girls Brain Development Trajectories
Differences between girls and boys, in terms of brain development, are much larger than differences between girls and boys in terms of height. In fact, as these investigators point out, there is no average difference in height between girls and boys between the ages of 10 and 12. And yet, there is a dramatic difference in the brain and in the trajectories of brain development (see their discussion of this point on page 1070 of their report in NeuroImage).

Differences between the brains of adult women compared with adult men are small. Differences between the brains of girls compared with boys are very large! A recent study from the Harvard University Medical School demonstrated this point even more dramatically with regard to the areas of the brain involved in emotional processing.

On many parameters of great relevance to education, the same principle is apparent. Differences between adult women and men are small in comparison with differences between differences between girls and boys. Here’s one example: How long can you sit still, be quiet and pay attention? If you compare 30-year-old men and 22-year-old women, you won’t find dramatic differences in how long women and men can sit still, be quiet and pay attention. But ask the same question about 6-year-olds: how long can the average 6-year-old boy sit still, be quiet and pay attention – compared with the average 6-year-old girl? Most teachers will tell you that the boy cannot sit still, be quiet and pay attention nearly as long as the average six-year-old girl. The boy starts squirming, fidgeting, getting restless. He may be diagnosed with attention-deficit disorder; medication may be prescribed. What those boys may really need is not medication, but a better understanding that girls and boys develop along different developmental trajectories. What is developmentally appropriate for a 6-year-old girl may not be developmentally appropriate for a 6-year-old boy.