From a new meta-analysis out of Cornell that reviewed 400 papers over the past 35 years:

"A major reason explaining why women are underrepresented not only in math-intensive fields but also in senior leadership positions in most fields is that many women choose to have children, and the timing of child rearing coincides with the most demanding periods of their career, such as trying to get tenure or working exorbitant hours to get promoted," said lead author Stephen J. Ceci, professor of human development at Cornell.