-- Harvard University President Lawrence H. Summers created a maelstrom when he suggested recently that innate differences in sex may explain why fewer women succeed in science and math careers.
He has since apologized for any misunderstanding his remarks may have caused. But is there any truth to the idea that men's and women's brains are, in fact, different?
Recent research suggests it may be so.
From the way we record information to how we process language to the size of our brains and different regions of the brain, clear differences have emerged through animal studies and the use of technology such as brain scanning.
And this is just the beginning.
Jill Goldstein, a professor of psychiatry at Harvard Medical School, argues that social climates have only recently made such research acceptable.
"When I was growing up, to say there were sex differences in the brain, you weren't even supposed to talk about it," said Goldstein. "I think we're living in a time now when we can look at what some of these differences are without saying they are necessarily deterministic."
Disease by Gender
If the differences aren't always deterministic, why bother looking for them? Goldstein explains, besides satisfying a long curiosity about possible biological explanations for male and female behavior, the research can boost our understanding of sex-specific diseases and possible ways to cure them.
Depression, for example, appears to be twice as common in women as in men while women with schizophrenia seem to suffer less cognitive difficulties than men with the condition.
Nearly all neurodevelopmental diseases are either more common in one gender or more severe among one gender, says Nancy Forger of the University of Massachusetts at Amherst. Other conditions, including arthritis, heart disease, even lung cancer also seem to be influenced by a person's gender.
"Clearly, if we can understand what's different about male and female brains, then we can understand why one sex is more susceptible to a disease," said Forger.
Research has long shown that men's brains are larger, on average, than women's — by about 100 grams. This may partly be due to the fact that men are larger than women, on average. Plus, what men have in volume, women make up in connections between brain cells.
It's unclear how this may translate into behavior, but some studies have shown that women may use more parts of their brain at once while men are more inclined to have focused responses.
Men More Forgetful?
Studies in people with damage to the left sides of their brains, for example, show that men with damage are less likely to be able to recover their ability to talk. The work, from researchers in Bonn, Germany, suggested that men's verbal abilities may stem mostly from the left side of the brain. Meanwhile, women with left brain damage usually retained some language skills.
This difference might partly explain why studies have shown that infant girls speak sooner and use more words than infant boys.
Other work has tapped functional MRIs -- scanning devices that measure blood flow and activity in the brains of conscious subjects. Drs. Ruben and Raquel Gur at the University of Pennsylvania have shown that women's brains light up in more areas than men's brains when given verbal and spatial tasks. This feature, they argue, may enhance women's ability to focus on many tasks at once.
Women may even use different pathways than men when thinking and encoding memories.
Turhan Canli, a Stanford psychologist, recently tested 12 men and 12 women in functional MRIs and showed that women encode memories using different pathways than what men use when recording memories. The women were later able to recall emotions of a memory more accurately than men, which could possibly stem from how their memories were encoded in the first place.
Other differences keep emerging, including variability in size of the different brain regions, including the hippocampus, the amygdala and certain brain cell clusters.
"At least 100 sex differences in male and female brains have been described so far," said Forger. "They keep cropping up in animal and human studies."
So what lies at the root of the male-female variability? In mammals, testosterone appears to be a main player.
Testosterone: A Brain-Shaper?
Forger's work in mice has shown that as mammals develop in the womb, testosterone and related hormones trigger cell death in some regions of the male brain and foster cell development in other regions. In this way, the hormone sculpts the male brain and how it will differ from the female version.
Remove or add testosterone to mice shortly after birth, and their brains develop according to the presence of the hormone, regardless of their sex.
Just as the Harvard president's remarks created a stir, this kind of research remains controversial, as does any work that looks for explanations for human behavior in the brain.
But most researchers looking into differences of the brain are quick to point out that there are many more differences in the brain just between individuals than between groups of people or between the sexes.
"Men and women are more the same than different in the brain -- without a question," said Forger. "But," she added, "little differences can go a long way."