Ability? Women, on average, score better than men in math. 45% of undergraduate math degrees go to women.
Choices? While it's true that women tend to prefer more life-related careers than men (i.e. biology rather than computer engineering), a disparity between women and men professors still exists in many STEM (science, technology, engineering, and mathematics) fields, not just the cold-hard-numbers professions.
Discrimination? One survey showed that in academics, women were offered plenty of opportunities for project funding and publishing.
The main factor here is children.
Childless women in university settings actually do very well, as do men with children. But for a woman who wants to raise a family, the university environment is not very family-friendly.
Imagine you're a young woman wanting to marry someday and start a family. You meet your one-and-only while you're working on a master's degree. But you have your heart set on teaching, so you decide to get your doctorate. It takes you less time than average, so you're done at age 29. Your biological clock is ticking, and you know you want to have more than one child. But you need experience. You apply for a teaching job, but these are scarce, so you look all over the country and finally land the interview you were hoping for. They hire you right away, but you don't want to wait any longer to start your family, and your husband doesn't, either. You know you won't get tenure as a full professor if you stop now, but you're not willing to wait six to ten more years, so you quit near the end of your first pregnancy, hoping that somehow, someday, you can pick up where you left off. "Only one in three women who accepts a fast-track university job before having a child ever becomes a mother," says the article, and it's no wonder.
It's most often the most time-intensive careers that keep mothers out. That's no surprise, considering how time-consuming it is to raise a family. This is especially true for women, who bear the biological brunt of having children. It's hard to work 12-hour days when you can barely walk up the stairs during a pregnancy. And if, for some strange child-bonding reason, moms want to take time off to raise their children, the unforgiving tenure clock makes it very difficult.
We can do better. The article has several suggestions on ways to make academics more family-friendly. Let's not let traditional clocks and requirements make us forget to accommodate the people who are raising the next generation of students, workers, and citizens.