When it comes to gender differences, everybody's an expert. But George Lazarus is a bit more expert than most. Although he doesn't study the subject formally, as a pediatrician in New York City he sees a lot of children, who are, after all, far better than adults at expressing their essential natures. One girl's parents, for instance, set out to raise her without "gender bias" that might hinder her success later in life. When she turned 3, they eschewed dolls and gave her toy trucks instead. The girl went off to her bedroom to play. When the parents checked up on her, they found her tucking the trucks in bed for the night. "Shhhh!" she said. "They're sleeping."
It's a story that Larry Summers, the beleaguered president of Harvard University, might appreciate. Summers caused a firestorm when he suggested several weeks ago that differences in "intrinsic aptitude" might be the principal reason the university has fewer females in the sciences and engineering than males; he lost a vote of no confidence in the Faculty of Arts and Sciences last week. Summers may be guilty of social indiscretion, but is he wrong, scientifically speaking? Does biology play no significant role in determining the talents and behaviors of men and women?
Considering the importance of the question, few studies have addressed it. Nevertheless, in recent years, scientists have been finding that the biological rift between men and women is larger than previously thought. To an extent few would have believed a few years ago, the center of gravity of scientific opinion on gender has begun to shift--and it's making everybody uncomfortable.
One of the most intriguing findings concerns the genetic differences between men and women. A study published last week in the journal Nature puts this difference at about 1 percent. Considering that the genetic makeup of chimpanzees and humans differs by only 1.5 percent, this is significant. "You could say that there are two human genomes, one for men and one for women," says Huntington Willard, a geneticist at Duke University and coauthor of the article. The study did not spell out exactly which genes do what. Rather, its results were like looking at the innards of two almost identical clocks and finding that in fact each has an altogether different arrangement of gears.
Scientists have long known that a person's sex is determined by two chromosomes, or bundles of genes--a woman inherits two X chromosomes, one from each parent, while a man inherits an X from mom and a Y from dad. For the past 40 years, scientists have thought that the extra X chromosome in females shuts down, while the other works alone. The Nature study, though, found that about 20 percent of the genes on the duplicate X chromosome--about 200 genes in all--remain active. Men, by contrast, have only one active X chromosome (plus a few genes on the puny Y chromosome). Not only are women genetically more complex and varied than men, they differ widely from one another.
Only a few years ago, scientists used to think that hormones were the primary mechanism of gender. The Y chromosome was assumed to do little but trigger a cascade of genes scattered among the other 22 human chromosomes, which ends with the production of the testes. Hormones still do a lot of the heavy lifting--with one crucial difference. Scientists have found that while hormones wreak havoc on just about every part of adolescent physiology, they have almost no effect on brain development. Studies of girls born in triplets, sandwiched in the womb between two brothers, show that although the girls acquire some masculine traits due to a heavier-than-normal dose of testosterone, their brains are unaffected. Genetic variations, on the other hand, have a huge impact on the brain. Down-syndrome boys, born with extra genes from chromosome 21, are cognitively impaired. When it comes to the brain, genes rule.
How, then, do female brains differ from male brains? Scientists are only beginning to address this question. So far, it seems clear that men and women think differently in significant ways. When navigating a maze, men tend to think spatially (go north for 200 meters and then turn left), while women look for landmarks. Brain scans of men and women engaged in rhyming words show that they use different brain circuits to perform the same task. Women also have 15 to 20 percent more gray matter (ordinary neurons) than men. And their white matter (long neurons that help the brain distribute its processing tasks) is concentrated at the juncture between the brain's left and right hemispheres, and may help women use both sides of their brain for language-related tasks.
There's also anecdotal evidence. On the Scholastic Aptitude Test, a college entry exam, women consistently score lower than men on the mathematics portion. (They do better on language skills, but still score slightly lower than men.) And then there are things like the makeup of the Harvard faculty. Such real-world evidence, of course, doesn't tell us what is cause and what is effect. To what extent does environment--education, upbringing, nutrition, exposure to stress, chemicals and so forth--play a role? Are boys slower to develop verbal skills because of their genes, or because they spend more time playing with trucks than talking with their friends?
Is Larry Summers right or wrong? At the moment, there's too little data to say. Even when scientists eventually come to understand the genetic clockwork, there's a good chance the answer won't be quite so simplistic. Individuals vary so widely in ability that any aggregate difference between men and women won't likely affect the ambitions of any aspiring scientist or playwright. Besides, genes can confer both advantages and disadvantages. The chances are pretty good that we haven't yet measured them all.