Head size correlates statistically with intelligence, a not-widely known fact - greater cranial volumes are linked to higher intelligence. The correlation, while small, has been reported in multiple studies. And because, on average, men's bodies are bigger than women's, their heads too are larger than women's. So, one might reasonably expect men to be more intelligent than women. This is not the case, however - men and women consistently score equally on intelligence tests. For neuroscientists, this paradox has long presented a puzzle. If head size correlates with intelligence, and women have smaller heads, why don't they have lower intelligence?
Researchers at the University of Pennsylvania Medical Center now report new findings that may help explain the conundrum: Women have a higher proportion of gray matter to cranial volume than men. Conversely, men have higher proportions of white matter and cerebrospinal fluid to cranial volume than women. Gray matter refers to the neuronal cell bodies and their dendrites, the short protrusions that communicate with immediately neighboring neurons in the brain. White matter refers to the longer axons, sheathed in a white fat called myelin, that reach out from neurons to more distant regions of the brain. Cerebrospinal fluid is the liquid in which the brain floats inside the head. Gray matter is where computation takes place, while white matter is responsible for communication between groups of cells in different areas of the brain.
"With these findings, we are beginning to get an explanation for the lack of intelligence disparity between men and women," says Ruben C. Gur, Ph.D., professor of psychology in psychiatry and lead author on the new study, which will appear in the May 15 issue of the Journal of Neuroscience. "Women have a higher percentage of tissue devoted to computation than men. Men have a greater proportion of tissue assigned to the transfer of information between distant regions."
The study replicated the findings associating better cognitive performance with larger volumes of brain tissue. The researchers determined, however, that with similar increases in volume, women gain more intelligence than men.
"Women's brains appear to be more efficient than men's in the sense that an equal increase in volume produces a larger increase in processing capacity in women than in men," notes Raquel E. Gur, M.D., Ph.D., professor of psychiatry and neurology and senior author on the study.
The study also offers a possible explanation for long-observed ability differences between men and women on specific types of cognitive tasks. Generally, women excel on verbal tasks, while men do better on spatial tasks. In the current study, these task-specific differences in ability were reconfirmed and correlated with the newly found proportional differences in gray and white matter in men and women. While higher volume of tissue correlated with better performance on both verbal and spatial intelligence tasks, the highest levels of spatial task performance required greater amounts of white matter than can be accommodated in the crania of most women.
"When we looked at the top performers for spatial tasks in our study - those performing better than one standard deviation above the average - there were nine men and only one woman," Ruben Gur says. "Of these nine men, seven had greater white-matter volumes than any of the women in the study. This suggests that, in order to be a super performer in that area, one needs more white matter than exists in most female brains."
The new data shed light on earlier observations concerning the corpus callosum, a large body of nerve fibers that connects the right and left hemispheres of the brain. Those studies showed that women have a relatively larger corpus callosum than men. The corpus callosum, however, is composed of white matter, the tissue type seen in this study at lower overall proportions in women than men in the brain, suggesting that evolution has placed a priority on this structure in women.
"The implication of women having more white matter connecting between the hemispheres of the brain is that they would have better communication between the different modes of perceiving and relating to the world," says Raquel Gur. "On the other hand, men would demonstrate a stronger concentration on working within any one of those modes."
Forty men and forty women, all healthy adults, volunteered to participate in the study. Gray and white matter percentages and overall cranial volumes were assessed using three-dimensional MRI imaging techniques. Cognitive performance was measured using tasks designed to examine different types of intellectual ability. An example of a spatial task given to the study participants involved viewing an array of lines fanning out at angles. Shown a second set of two shorter lines at similar angles, the participants were asked to identify which lines in the original fan array matched the angles of the shorter lines. One verbal intelligence task involved extrapolations of word relationships. Some examples: "sailor is to navy as soldier is to: a) gun, b) cap, c) hill, d) army" and "thoughts are to brains as steam is to: a) water, b) vapor, c) boiling, d) heat."
In addition to Ruben and Raquel Gur, the coauthors on the study were Bruce I. Turetsky, Mie Matsui, Michelle Yan, Warren Bilker, and Paul Hughett. Funding for the research was provided by the National Institutes of Health.
The University of Pennsylvania Medical Center's sponsored research and training ranks second in the United States based on grant support from the National Institutes of Health, the primary funder of biomedical research and training in the nation -- $201 million in federal fiscal year 1998. In addition, the institution continued to maintain the largest absolute growth in funding for research and training among all 125 medical schools in the country since 1991. News releases from the University of Pennsylvania Medical Center are available to reporters by direct e-mail, fax, or U.S. mail, upon request.