Gender? It's A Gray Area.

According to Liberman, most research concludes that men and women use an equal number of words in a day. Liberman also says that the study that Brizendine cites to show that women talk twice as fast as men "doesn't support this at all, and in fact I found other research that showed men speak slightly faster."

These claims, Liberman says, "simply violate common sense." Gender differences, outside of contests of physical strength, when they are found at all, are mostly only a matter of a statistical point or two. Rarely is one sex two or three times better or worse at anything -- except, well, giving birth. "It reminds me of the myth that Eskimos have hundreds of words for snow," Liberman says. "They don't. The various Eskimo languages and English have the same number."

In a follow-up e-mail exchange about her sources, Brizendine stated, for example, that she found the sex-on-the-brain figure in the work of John Bancroft, formerly head of the Kinsey Institute. But in the FAQ section of the Kinsey Institute Web site, it reports that 54 percent of men think about sex every day or several times a day, while 19 percent of women think about sex every day or several times a day -- or so they say. (These figures are included in 1994's "Sex in America: A Definitive Survey" from a University of Chicago research team.)

Sloppy or not, Brizendine is on to something. The last decade of science, especially the ability to scan the brain while it performs simple tasks (the human subjects lie in magnetic resonance imaging tubes), is providing evidence of gender differences in the architecture and activity of the noggin. But a lot of it is pretty subtle and the meanings still obscure.

"There does appear to be more than one brain design," agrees Richard Haier, professor of psychology at the University of California, Irvine, whose own studies have shown that women possess, on average, a greater density of white matter, and men more gray matter (gray is the cell bodies of neurons; white is the stuff that connects them).

But it is equally interesting, Haier says, that male and female brains arrive at the same destination. IQ scores are essentially equal between the sexes. (Though in SATs, males consistently score slightly higher; some argue the tests are male-biased.)

Phrenologists at the turn of the last century, upon discovering that the average male brain is about 9 percent larger than the female (true -- and attributable to body size), shouted "Eureka!" and put the observation to work to justify little lady stereotypes about women. Post-1960s researchers tended to see men and women as neurologically identical, and any differences as culturally learned (hence the short-lived trend of giving boys dolls and girls trucks).

The pendulum is swinging again, and the "gender genre" is currently as hot as a flash in perimenopause, with "The Female Brain" sharing shelf space with other popular science books such as "The Mommy Brain" (eeeek, it shrinks! ), "The Essential Difference" (autism is an extreme form of maleness) and "Why Gender Matters" (immediately enroll your kid in a single-sex school). It is a contentious brew of biological determinism, stirring up bugaboos about gender "traits" and "strengths" that a feminist may say always end up conveniently relegating women to roles as nurturing caregiver (vs. Condoleezza Rice).

Look no further than last year's furor when Lawrence Summers, now former president of Harvard, suggested that the reason women do not occupy the top ranks of science and engineering has something to do with their brains. Brizendine says that Summers was actually right and wrong. Her take: When boys and girls enter their teens, their math and science abilities are equal. "But as estrogen floods the female brain," she says, "females start to focus intensely on their emotions and on communication." Talking on the phone, furious text-messaging, dressing-room confabbing. "At the same time, as testosterone takes over the male brain, boys grow less communicative and become obsessed about scoring -- in games, and in the back seat of the car. At the point when boys and girls begin deciding the trajectories of their careers, girls start to lose interest in pursuits that require more solitary work and fewer interactions with others, while boys can easily retreat alone to their rooms for hours of computer time." (Noted: Most science is done by teams of researchers.)

Brizendine says she was initially torn about highlighting the differences between male and female brains. Her politics are Bay Area liberal, and she's as politically correct as anyone.

"I thought, oh boy, this isn't going to be good for women," Brizendine says. "I struggled with it. I was very cognizant that some of these will be used against women. But I decided I would go with the data and the science. Because actually this is very good for women in pointing out their innate skill sets and strengths."

But some of her critics say that what Brizendine did was overstate the science. In part, it may be the style that Brizendine adopts when she speaks and writes. When science looks for differences, it finds them in the average male and average female -- meaning that if six in 10 women show an advantage in one area, so do four in 10 men. But this gets lost in the prose.

For example, according to Brizendine, "during puberty, a girl's entire biological raison d'etre is to become sexually desirable" and "they are almost exclusively interested in their appearance." The female brain, she writes, is "a machine . . . that is built for connection. That's the main job of the girl brain, and that's what it drives a female to do from birth."

Brizendine, in part, blames her publisher for requesting that she edit out her repeated use of the words "typical" and "average."

"These are stereotypes, and stereotypes die hard," says Janet Hyde, psychology professor at the University of Wisconsin at Madison, whose own studies find that instead of vast differences between the sexes, males and females are more psychologically similar. Hyde reviewed 46 so-called meta-analyses that examined gender differences (across a wide swath of categories, such as math, reading, sexuality, happiness, assertiveness, etc.). Her study showed that "in most areas there are either no differences or very small differences," she says, with a few exceptions: Men are more physically aggressive, better at throwing; they masturbate more often, have more relaxed attitudes about casual sex. But Hyde prefers that science adopt a "gender similarities hypothesis" rather than search for differences.

Brizendine sees her point. "What we are finding is small differences in the male and female brain," she says. "But medical science tells us that small differences can have big outcomes." Think of cancer, she says; the slightest alteration of cells can have the most lethal consequences.

In a 2001 National Academy of Sciences report, the authors write: "Sex matters. Sex, that is, being male or female, is an important basic human variable that should be considered when designing and analyzing studies in all areas and at all levels of biomedical and health-related research."

This is going to be a bumpy ride.