The latest study on women in science by Stephen Ceci and Wendy Williams of Cornell doesn't break completely uncharted ground. But it does build a really solid case in numbers for why women's life decisions contribute to why they under-perform men in scientific fields.
The dilemma: More than half of MD degrees, PhDs in life sciences and social sciences, and roughly three quarters of PhDs in psychology and veterinary medicine, are held by women. However, in the top 100 universities, only a fraction of tenure-track positions in "math-intensive fields" are held by women. Less that ten percent of full professors are women. Bias gets lots of attention in the literature. Ceci and Williams say it just doesn't explain a gap this big.
This situation is caused mainly by women's choices, both freely made and constrained by biology and society, such as choices to defer careers to raise children, follow spouses' career moves, care for elderly parents, limit job searches geographically, and enhance work-home balance. Some of these choices are freely made; others are constrained and could be changed.