Neuroscience For Kids

He Brains, She Brains

Bigger - Stronger - Faster...are there really any differences between female brains and male brains? Differences between the brains of men and women have generated considerable scientific and public interest. If there are differences in the way that men and women behave, then it is reasonable to suppose that their brains have something to do these behavioral differences. Just what are these differences and where in the brain might these differences be located?

For hundreds of years, scientists have searched for differences between the brains of men and women. Early research showing that male brains were larger than female brains was used to "prove" that male brains were superior to female brains. Of course, this "proof" is NOT so simple and straight forward as you will see. Nevertheless, even today, there is plenty of controversy about the differences in the brains of men and women. Not only from an anatomical point of view, but also from a functional point of view - in other words, just what do the differences in the brains mean?

Hormones that are present during a baby's development will affect the brain and determine whether the brain will be female or male. Studies that have looked at differences in the brains of males and females have focused on:

Total Brain Size
The Corpus Callosum
The Hypothalamus

Differences in Total Brain Size?

Almost all studies show that at birth, a boy's brain is bigger than a girl's brain. At birth, the average brain of boys is between 12-20% larger than that of girls. The head circumference of boys is also larger (2%) than that of girls. However, when the size of the brain is compared to body weight at this age, there is almost no difference between boys and girls. So, a girl baby and a boy baby who weigh the same will have similar brain sizes.

In adults, the average brain weight in men is about 11-12% MORE than the average brain weight in women. Men's heads are also about 2% bigger than women's. Remember though, men on average weigh more than women and that absolute brain size may not be the best measure of intelligence. Many behavioral differences have been reported for men and women. For example, it has been said that women are better in certain language abilities and men are better in certain spatial abilities. Many studies have tried to find differences in the right and left cerebral hemispheres to suggest that male and female brains are different. However, few of these experiments have found meaningful differences between men and women. If fact, there are many similarities between the cerebral hemispheres of men and women.

Brain Weights
graph
Data from Dekaban, A.S. and Sadowsky, D., Changes in brain weights during the span of human life: relation of brain weights to body heights and body weights, Ann. Neurology, 4:345-356, 1978)

Differences in the Corpus Callosum?

mri The major pathway that connects the right and left cerebral hemispheres is called the corpus callosum. (The corpus callosum is the fiber tract made up of 200-250 million axons that is cut in split brain patients.) Some claims have been made that the corpus callosum is bigger and more developed in women than in men. These claims have even been reported in the popular media (Time Magazine, Jan. 20, 1992, pp. 36-42; Newsweek Magazine, March 27, 1995, pp. 51). However, other studies have told a different story. Using magnetic resonance imaging methods, some researchers have found no differences in the size of the corpus callosum of men and women or that the corpus callosum is larger in men than in women (Allen et al., 2003).

Differences in the Hypothalamus?

The hypothalamus is one area of the brain with well-documented differences between men and women. Two areas of the hypothalamus, the preoptic area and the suprachiasmatic nucleus, have clear differences in female and male brains.

Preoptic Area of the Hypothalamus: This area of the hypothalamus is involved in mating behavior. In males of several species including humans, the preoptic area is greater in volume, in cross-sectional area and in the number of cells. In men, this area is about 2.2 times larger than in women and contains 2 times more cells. Apparently, the difference in this area is only apparent after a person is 4 years old. At 4 years of age, there is a decrease in the number of cells in this nucleus in girls. The exact function of this nucleus in behavior is not fully known.

Suprachiasmatic Nucleus of the Hypothalamus: This area of the hypothalamus is involved with circadian rhythms and reproduction cycles. The only difference between women and men in this area is one of shape: in males, this nucleus is shaped like a sphere; in females it is more elongated. However, the number of cells and volume of this nucleus are not different in men and women. It is possible that the shape of the suprachiasmatic nucleus influences the connections that this area makes with other areas of the brain, especially the other areas of the hypothalamus.

Women and Men - Boys and Girls

The behavioral and neurological differences between men and women require further study. Perhaps new studies will find neuroanatomical differences that explain some of the complex differences between male and female behavior. However, from a review of the current scientific evidence, it appears that differences in many cognitive behaviors (for example, memory) are related more to individual differences between people than to whether people are female or male.

Hear IT!

Hypothalamus | Corpus Callosum

More about the possible differences between male and female brains:

"His Brain, Her Brain" by Larry Cahill in Scientific American, May, 2005.
Brain imaging study of spatial memory
Allen, J.S., Damasio, H., Grabowskia, T.J., Brussa, J. and Zhang, W., Sexual dimorphism and asymmetries in the gray?white composition of the human cerebrum, NeuroImage, 18:880-894, 2003.