Who Says A Woman Can't Be Einstein?

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ut most studies agree that men's brains are about 10% bigger than women's brains overall. Even when the comparison is adjusted for the fact that men are, on average, 8% taller than women, men's brains are still slightly bigger. But size does not predict intellectual performance, as was once thought. Men and women perform similarly on IQ tests. And most scientists still cannot tell male and female brains apart just by looking at them.

Recently, scientists have begun to move away from the obsession with size. Thanks to new brain-imaging technology, researchers can get a good look at the living brain as it functions and grows. Earlier studies relied on autopsies or X rays—and no one wanted to expose children or women, who might be pregnant, to regular doses of radiation.

The deeper you probe, the more interesting the differences. Women appear to have more connections between the two brain hemispheres. In certain regions, their brain is more densely packed with neurons. And women tend to use more parts of their brain to accomplish certain tasks. That might explain why they often recover better from a stroke, since the healthy parts of their mind compensate for the injured regions. Men do their thinking in more focused regions of the brain, whether they are solving a math problem, reading a book or feeling a wave of anger or sadness.

Indeed, men and women seem to handle emotions quite differently.

While both sexes use a part of the brain called the amygdala, which is located deep within the organ, women seem to have stronger connections between the amygdala and regions of the brain that handle language and other higher-level functions. That may explain why women are, on average, more likely to talk about their emotions and men tend to compartmentalize their worries and carry on. Or, of course, it may not.

"Men and women have different brain architectures, and we don't know what they mean," says Haier. By administering IQ tests to a group of college students and then analyzing scans of their brain structure, Haier's team recently discovered that the parts of the brain that are related to intelligence are different in men and women. "That is in some ways a major observation, because one of the assumptions of psychology has been that all human brains pretty much work the same way," he says. Now that we know they don't, we can try to understand why some brains react differently to, say, Alzheimer's, many medications and even teaching techniques, Haier says.

Even more interesting than the brain's adult anatomy might be the journey it takes to get there. For 13 years, psychiatrist Jay Giedd has been compiling one of the world's largest libraries of brain growth. Every Tuesday evening, from 5 o'clock until midnight, a string of children files into the National Institutes of Health outside Washington to have their brains scanned. Giedd and his team ease the kids through the MRI procedure, and then he gives them a brain tour of their pictures—gently pointing out the spinal cord and the corpus callosum, before offering them a copy to take to show-and-tell.

Most of the kids are all business. Rowena Avery, 6, of Sparks, Nev., arrived last week with a stuffed animal named Sidewalk and stoically disappeared into the machine while her mom, dad and little sister watched. In preparation, she had practiced at home by lying very still in the bathtub. Her picture came out crystal clear. "The youngest ones are the best at lying still. It's kind of surprising," Giedd says. "It must be because they are used to hiding in kitchen cabinets and things like that." Among the girls in Giedd's study, brain size peaks around age 11 1/2.

For the boys, the peak comes three years later. "For kids, that's a long time," Giedd says. His research shows that most parts of the brain mature faster in girls. But in a 1999 study of 508 boys and girls, Virginia Tech researcher Harriet Hanlon found that some areas mature faster in boys. Specifically, some of the regions involved in mechanical reasoning, visual targeting and spatial reasoning appeared to mature four to eight years earlier in boys. The parts that handle verbal fluency, handwriting and recognizing familiar faces matured several years earlier in girls.