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Among the girls in Giedd's study, brain size peaks around age 111/2. 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.
Monkeys are among our most trusted substitutes in brain research. This week a study in the journal Behavioral Neuroscience shows that stage of life is also important in male and female rhesus monkeys. In a sort of shell game, young male monkeys proved better at finding food after they saw it hidden on a tray--suggesting better spatial memory. But they peaked early. By old age, male and female monkeys performed equally well, according to the study, which was led by Agnès Lacreuse at the Yerkes National Primate Research Center. All of which suggests that certain aptitudes may not be that different between males and females. It just depends on when you test them. (We'll have more to say about those monkeys in just a bit.)
LESSON 2: THE SEGREGATION OF THE SENSES
SO HOW DO WE EXPLAIN WHY, IN STUDY after study, boys and men are still on average better at rotating 3-D objects in their minds? As for girls and women, how do we explain why they tend to have better verbal skills and social sensitivities?
The most surprising differences may be outside the brain. "If you have a man and a woman looking at the same landscape, they see totally different things," asserts Leonard Sax, a physician and psychologist whose book Why Gender Matters came out last month. "Women can see colors and textures that men cannot see. They hear things men cannot hear, and they smell things men cannot smell." Since the eyes, ears and nose are portals to the brain, they directly affect brain development from birth on.
In rats, for example, we know that the male retina has more cells designed to detect motion. In females, the retina has more cells built to gather information on color and texture. If the same is true in humans, as Sax suspects, that may explain why, in an experiment in England four years ago, newborn boys were much more likely than girls to stare at a mobile turning above their cribs. It may also help explain why boys prefer to play with moving toys like trucks while girls favor richly textured dolls and tend to draw with a wider range of colors, Sax says.
Likewise, women's ears are more sensitive to some noises. Baby girls hear certain ranges of sound better. And the divergence gets even bigger in adults. As for smell, a study published in the journal Nature Neuroscience in 2002 showed that women of childbearing age were many times more sensitive than men to several smells upon repeated exposure. (Another study has found that heterosexual women have the most sensitive smell and homosexual men have the least.)