Researchers reveal sex differences in the brain's form and function.
Reviewed By Brunilda Nazario
Recent studies highlight a long-held suspicion about the brains of males and females. They're not the same. So how does the brain of a female look and function differently from a male's brain, and what accounts for these differences?
Disparities Start Early in Life
Scientists now know that sex hormones begin to exert their influence during development of the fetus. A recent study by Israeli researchers that examined male and female brains found distinct differences in the developing fetus at just 26 weeks of pregnancy. The disparities could be seen when using an ultrasound scanner. The corpus callosum -- the bridge of nerve tissue that connects the right and left sides of the brain -- had a thicker measurement in female fetuses than in male fetuses.
Observations of adult brains show that this area may remain stronger in females. "Females seem to have language functioning in both sides of the brain," says Martha Bridge Denckla, PhD, a research scientist at Kennedy Krieger Institute.
Consider these recent findings. Researchers, using brain imaging technology that captures blood flow to "working" parts of the brain, analyzed how men and women process language. All subjects listened to a novel. When males listened, only the left hemisphere of their brains was activated. The brains of female subjects, however, showed activity on both the left and right hemispheres.
This activity across both hemispheres of the brain may result in the strong language skills typically displayed by females. "If there's more area dedicated to a set of skills, it follows that the skills will be more refined," says David Geary, PhD, professor of psychological sciences at the University of Missouri.
As a whole, girls outperform boys in the use of language and fine motor skills until puberty, notes Denckla. Boys also fall prey to learning disabilities more frequently than girls. "Clinics see a preponderance of boys with dyslexia," Denckla tells WebMD. ADHD also strikes more boys than girls. The symptoms displayed by girls and boys with ADHD differ, too. Girls with ADHD usually exhibit inattention, while affected boys are prone to lack of impulse control. But not all differences favor girls.
Boys generally demonstrate superiority over female peers in areas of the brain involved in math and geometry. These areas of the brain mature about four years earlier in boys than in girls, according to a recent study that measured brain development in more than 500 children. Researchers concluded that when it comes to math, the brain of a 12-year-old girl resembles that of an 8-year-old boy. Conversely, the same researchers found that areas of the brain involved in language and fine motor skills (such as handwriting) mature about six years earlier in girls than in boys.
So, do these sex differences even out over time?
Females and males maintain unique brain characteristics throughout life. Male brains, for instance, are about 10% larger than female brains. But bigger doesn't necessarily mean smarter.
Disparities in how certain brain substances are distributed may be more revealing. Notably, male brains contain about 6.5 times more gray matter -- sometimes called 'thinking matter" -- than women. Female brains have more than 9.5 times as much white matter, the stuff that connects various parts of the brain, than male brains. That's not all. "The frontal area of the cortex and the temporal area of the cortex are more precisely organized in women, and are bigger in volume," Geary tells WebMD. This difference in form may explain a lasting functional advantage that females seem to have over males: dominant language skills.
How Males and Females Use Mental Skills
Geary suggests that women use language skills to their advantage. "Females use language more when they compete. They gossip, manipulate information," he says. Geary suggests that this behavior, referred to as relational aggression, may have given females a survival advantage long ago. "If the ability to use language to organize relationships was of benefit during evolutionary history, and used more frequently by women, we would expect language differences to become exaggerated," he tells WebMD. Women also use language to build relationships, theorizes Geary. "Women pause more, allow the other friend to speak more, offer facilitative gestures," he says.
When it comes to performing activities that require spatial skills, like navigating directions, men generally do better. "Women use the cerebral cortex for solving problems that require navigational skills. Men use an entirely different area, mainly the left hippocampus -- a nucleus deep inside the brain that's not activated in the women's brains during navigational tasks," Geary tells WebMD. The hippocampus, he explains, automatically codes where you are in space. As a result, Geary says: "Women are more likely to rely on landmark cues: they might suggest you turn at the 7-11 and make a right at the church, whereas men are more likely to navigate via depth reckoning -- go east, then west, etc."
While the brain allows us to think, it also drives our emotions. It may not come as a surprise, then, that the ability to identify and control emotions varies between sexes.
"Women are faster and more accurate at identifying emotions," says Ruben Gur, PhD, a neurologist at the University of Pennsylvania. Studies have shown women to be more adept than men at encoding facial differences and determining changing vocal intonations.
Women, as a whole, may also be better than men at controlling their emotions. Gur and colleagues at the University of Pennsylvania recently discovered that sections of the brain used to control aggression and anger responses are larger in women than in men.
Gender-Based Skills Overlap and Complement
Recent studies that highlight sex-associated brain differences may lead us to believe that men and women have little in common upstairs. That's not the case. "Men and women do have lots of brain areas that are the same," Geary tells WebMD. Moreover, members of both sexes excel at skills that are commonly labeled gender specific. "All of these things have overlapping distributions. There are many women with better-than-average spatial skills, and men with good writing skills," Geary says.
Some researchers believe that nurturing one's brain can enhance what nature has provided. Consider, for instance, the general superiority of males' spatial abilities. "There's a lot of evidence that we build up our brain's representation of space by moving through it," Denckla tells WebMD. As anyone who spends a significant time around children knows, boys tend to get a lot more practice "moving through space" -- chasing a ball, for instance -- than girls do. "My hypothesis is that we could possibly erase this difference if we pushed girls out into the exploratory mode," Denckla says. She predicts that as more and more girls engage in sports traditionally reserved for boys, like soccer, the data on spatial ability will show fewer disparities between females and males.
Others believe brain variations between sexes are for the best. "Most of these differences are complementary. They increase the chances of males and females joining together. It helps the whole species," Gur says.
Published April 11, 2005.
SOURCES: Martha Bridge Denckla, PhD, research scientist, Kennedy Krieger Institute. David Geary, PhD, professor of psychological sciences, University of Missouri. Ruben Gur, PhD, neurologist, University of Pennsylvania.
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