The Study of Sex Differences Over Time

Do sex differences matter? How large are the differences between girls and boys? To what extent are those differences hardwired? 

Many people are uncomfortable with these questions. There are good reasons for that discomfort. For at least two thousand years, men have insisted that women and men are different, and those differences have been used as a justification for keeping women down, and for excluding women from positions of power in business and in politics. More fundamentally, men have a long history of using all sorts of imaginary science to create an order of rank, with men at the top. Here’s an extreme example, from the Summa Theologica of St. Thomas Aquinas

As regards the individual nature, woman is defective and misbegotten, for the active force in the male seed tends to the production of a perfect likeness in the masculine sex; while the production of woman comes from defect in the active force or from some material indisposition, or even from some external influence, such as that of a south wind, which is moist . . .

Medical doctors have also been guilty of dispensing sexism-disguised-as-science. In 1873, Harvard professor Dr. Edward Clarke wrote a best-selling book in which he asserted that women should not be allowed to attend college. If young women studied too much, he asserted, they would divert blood away from the uterus to the brain, rendering themselves “irritable and infertile.”

Dr. Clarke’s book was not scientific; from today’s perspective, it is obviously merely sexism pretending to be science. Thirty years after Dr. Clarke’s book appeared, a German neurologist wrote a slightly more serious book about sex differences in the human brain. This neurologist, Paul Julius Möbius, carefully measured the volume of the skulls of women and men. He had a large collection of skulls, so he was able to control for parameters of age and weight. He would compare the skull of a man who died at age 60 with the skull of a woman the same height and the same age. By pouring sand into each skull, and measuring how much sand he could get into each skull, he was able to measure the volume – the cranial capacity – of each skull. 

Using this method, he found that the cranial capacity of the skull in a man averaged about 8% more than in the skull of a woman the same age and height. He concluded that women are “physiologically weak-minded.”

His measurements turn out to be accurate, but his conclusion was certainly wrong. The volume of a man’s skull is indeed about 8% larger than the volume of a woman’s skull. However, there are other differences of which Möbius was unaware. For example:

  • More dead space in the male brain. There are large holes in the brain, called the lateral ventricles. The ventricles store cerebrospinal fluid, the water which cushions the brain against injury. These holes in the brain are about 20% larger in men than in women. This difference can’t be explained by height; even in 10-year-old boys, who are the same height (on average) as 10-year-old girls, these holes in the brain are still about 20% bigger, according to the world’s largest study of brain development in children.
  • Women have proportionately more gray matter. The cerebral cortex is significantly thicker in women than in men.  (The cerebral cortex is where most of our complex thinking takes place.) When you adjust for size differences between women and men, the differences become even more dramatic, with the cortex being thicker in many areas in women’s brains compared with men. The red areas in the figure at right (from this recent report from UCLA) shows the areas of the brain where the cortex is thicker in women. There is no region of the brain where the cortex is thicker in men than in women.

The emerging reality is that the brain is organized differently in females compared with males. Consider a recent study from University of California Irvine demonstrating that intelligence is represented differently in the brains of men compared with women. In this study, investigators studied the brains of high-IQ women and compared them with low-IQ women. They then compared high-IQ men with low-IQ men. They found consistent differences in the organization of the brain of high-IQ women compared with low-IQ women; and likewise, they found consistent differences in the organization of the brain of high-IQ men compared with low-IQ men. But there was no overlap in the differences between sexes. In other words, the brain features which distinguished high-IQ men compared with low-IQ men were completely different from the features which distinguished high-IQ women compared with low-IQ women. Likewise, in 2008, researchers at UCLA reported that at least some regions of the cerebral cortex appears to be organized in a fundamentally different way in men and women

The great danger here is to construct an order of rank where none exists. Yes, women’s brains appear to be built differently from men’s. But differences do NOT imply an order of rank. Apples and oranges are different; that doesn’t mean apples are better than oranges. Ovaries and testicles are different; that doesn’t mean ovaries are better than testicles. Girls and boys are different; that doesn’t mean girls are better or smarter than boys, or vice versa.

Setting the Record Straight

While research examining sex differences does not support the notion of an order of rank between males and females, there do appear to be distinct differences that influence how boys and girls experience the typical school classroom; how they understand instructions to clean the room; and how they respond to emotionally arousing situations.  Here we share some of the current science on sex differences as it pertains to education and parenting, so that parents and others who care about children can better understand the relevant issues. As you will see throughout this special edition of, ignoring the differences often disadvantages both girls and boys.


Take me to the articles on gender differences.