It's not lack of math keeping women from careers in science and engineering, nor is it the lack of other college-prep classes, say two sociologists in a new book. Motherhood is the problem.

In their book Women in Science: Career Processes and Outcomes, Kimberlee Shauman of UC Davis and Yu Xie of University of Michigan document the tremendous progress of women in the scientific labor force over the past four decades. But there's a critical barrier that keeps women from advancing further.

"We find that it is not marriage per se that hampers women's career development," Shauman and Xie say. "Rather, married women appear to be disadvantaged only if they have children."

Using multiple sources of national data, they look at differences in women's and men's education, career transitions following completion of undergraduate and graduate education, participation in the labor force, earnings, promotion, geographic mobility, immigration and research productivity.

Relative to their male counterparts, fewer women seek science and engineering degrees. The women who do graduate at a ratio equal to men. But after college, once they have children, women tend to participate less in the labor force, are less geographically mobile and less likely to be promoted.

The authors also found that women immigrants are more disadvantaged than native-born women in employment.

In addition, Shauman and Xie say, compared to other careers where gender inequality has declined more rapidly, scientific careers by their very nature hinder a balance between family and work.

"More than other careers, women in science careers do not bounce back from periods of inactivity for such things as childbearing," Shauman says.

That's because research often cannot be put on hold for maternity leave, progress in research is dependent upon a scientist's success in securing funding year after year, and science careers require extensive and intensive investments of time in education during prime child-bearing years.