WHEN IT COMES to the huge and persistent gender gap in science and technology jobs, the finger of blame has pointed in many directions: sexist companies, boy-friendly science and math classes, differences in aptitude. Women make up almost half of today's workforce, yet hold just a fraction of the jobs in certain high-earning, high-qualification fields. They constitute 20 percent of the nation's engineers, fewer than one-third of chemists, and only about a quarter of computer and math professionals. Over the past decade and more, scores of conferences, studies, and government hearings have been directed at understanding the gap. It has stayed in the media spotlight thanks in part to the high-profile misstep of then-Harvard president Larry Summers, whose loose comment at a Harvard conference on the topic in 2005 ultimately cost him his job. Now two new studies by economists and social scientists have reached a perhaps startling conclusion: An important part of the explanation for the gender gap, they are finding, are the preferences of women themselves. When it comes to certain math- and science-related jobs, substantial numbers of women - highly qualified for the work - stay out of those careers because they would simply rather do something else. One study of information-technology workers found that women's own preferences are the single most important factor in that field's dramatic gender imbalance. Another study followed 5,000 mathematically gifted students and found that qualified women are significantly more likely to avoid physics and the other "hard" sciences in favor of work in medicine and biosciences. It's important to note that these findings involve averages and do not apply to all women or men; indeed, there is wide variety within each gender. The researchers are not suggesting that sexism and cultural pressures on women don't play a role, and they don't yet know why women choose the way they do. One forthcoming paper in the Harvard Business Review, for instance, found that women often leave technical jobs because of rampant sexism in the workplace. But if these researchers are right, then a certain amount of gender gap might be a natural artifact of a free society, where men and women finally can forge their own vocational paths. And understanding how individual choices shape the gender balance of some of the most important, financially rewarding careers will be critical in fashioning effective solutions for a problem that has vexed people for more than a generation. A few years ago, Joshua Rosenbloom, an economist at the University of Kansas, became intrigued by a new campaign by the National Science Foundation to root out what it saw as pervasive gender discrimination in science and engineering. The agency was spending $19 million a year to encourage mentoring programs, gender-bias workshops, and cooperative work environments.

Rosenbloom had no quarrel with the goal of gender equity. But as he saw it, the federal government was spending all that money without any idea what would work, because there was no solid data on what caused the disparity between men and women in scientific fields. To help answer the question, Rosenbloom surveyed hundreds of professionals in information technology, a career in which women are significantly underrepresented. He also surveyed hundreds in comparable careers more evenly balanced between men and women. The study examined work and family history, educational background, and vocational interests. The results were striking. The lower numbers of women in IT careers weren't explained by work-family pressures, since the study found computer careers made no greater time demands than those in the control group. Ability wasn't the reason, since the women in both groups had substantial math backgrounds. There was, however, a significant difference in one area: what the men and women valued in their work. Rosenbloom and his colleagues used a standard personality-inventory test to measure people's preferences for different kinds of work. In general, Rosenbloom's study found, men and women who enjoyed the explicit manipulation of tools or machines were more likely to choose IT careers - and it was mostly men who scored high in this area. Meanwhile, people who enjoyed working with others were less likely to choose IT careers. Women, on average, were more likely to score high in this arena. Personal preference, Rosenbloom and his group concluded, was the single largest determinative factor in whether women went into IT. They calculated that preference accounted for about two-thirds of the gender imbalance in the field. The study was published in November in the Journal of Economic Psychology. It may seem like a cliche - or rank sexism - to say women like to work with people, and men prefer to work with things. Rosenbloom acknowledges that, but says that whether due to socialization or "more basic differences," the genders on average demonstrate different vocational interests. "It sounds like stereotypes," he said in an interview, "but these stereotypes have a germ of truth." In the language of the social sciences, Rosenbloom found that the women were "self-selecting" out of IT careers. The concept of self-selection has long interested social scientists as an explanation for how groups sort themselves over time. Since human beings are heterogeneous, self-selection predicts that when offered a menu of options and freedom of choice, people will make diverse choices and sort themselves out in nonrandom ways. In other words, even given the same opportunities, not everybody will do the same thing - and there are measurable reasons that they will act differently from one another. The concept of self-selection sets off alarms for many feminists. It seems to suggest that women themselves are responsible for the gender gap. It can also be an excuse for minimizing the role of social forces, including discrimination in the classroom and the workplace.

But self-selection has also emerged as the chief explanation in other recent studies of gender imbalance, including a long-term survey done by two Vanderbilt researchers, Camilla Persson Benbow and David Lubinski. Starting more than 30 years ago, the Study of Mathematically Precocious Youth began following nearly 2,000 mathematically gifted adolescents, boys and girls, tracking their education and careers in ensuing decades. (It has since been expanded to 5,000 participants, many from more recent graduating classes.) Both men and women in the study achieved advanced credentials in about the same numbers. But when it came to their career paths, there was a striking divergence. Math-precocious men were much more likely to go into engineering or physical sciences than women. Math-precocious women, by contrast, were more likely to go into careers in medicine, biological sciences, humanities, and social sciences. Both sexes scored high on the math SAT, and the data showed the women weren't discouraged from certain career paths. The survey data showed a notable disparity on one point: That men, relative to women, prefer to work with inorganic materials; women, in general, prefer to work with organic or living things. This gender disparity was apparent very early in life, and it continued to hold steady over the course of the participants' careers. Benbow and Lubinski also found something else intriguing: Women who are mathematically gifted are more likely than men to have strong verbal abilities as well; men who excel in math, by contrast, don't do nearly as well in verbal skills. As a result, the career choices for math-precocious women are wider than for their male counterparts. They can become scientists, but can succeed just as well as lawyers or teachers. With this range of choice, their data show, highly qualified women may opt out of certain technical or scientific jobs simply because they can. These studies looked at different slices of the working world, but agree that in a world in which men and women both have freedom of choice, they tend to choose differently. They have a provocative echo in the conclusions of Susan Pinker, a psychologist and columnist for the Toronto Globe and Mail. In her controversial new book, "The Sexual Paradox: Men, Women, and the Real Gender Gap," Pinker gathers data from the journal Science and a variety of sources that show that in countries where women have the most freedom to choose their careers, the gender divide is the most pronounced. The United States, Norway, Switzerland, Canada, and the United Kingdom, which offer women the most financial stability and legal protections in job choice, have the greatest gender split in careers. In countries with less economic opportunity, like the Philippines, Thailand, and Russia, she writes, the number of women in physics is as high as 30 to 35 percent, versus 5 percent in Canada, Japan, and Germany.