The Truth About Gender; The rift between the sexes just got a whole lot bigger. A new study has found that women and men differ genetically almost as much as humans differ from chimpanzees.

Byline: Fred Guterl

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 ...