Some very important findings on why women are underrepresented in math and science jobs:

We know that women are underrepresented in math and science jobs. What we don't know is why it happens.

There are various theories, and many of them focus on childhood. Parents and toy-makers discourage girls from studying math and science. So do their teachers. Girls lack role models in those fields, and grow up believing they wouldn't do well in them.

All these factors surely play some role. A new study points to the influence of teachers' unconscious biases, but it also highlights how powerful a little encouragement can be. Early educational experiences have a quantifiable effect on the math and science courses the students choose later, and eventually the jobs they get and the wages they earn.

The effect is larger for children from families in which the father is more educated than the mother and for girls from lower-income families, according to the study, published this week by the National Bureau of Economic Research.

The pipeline for women to enter math and science occupations narrows at many points between kindergarten and a career choice, but elementary school seems to be a critical juncture. Reversing bias among teachers could increase the number of women who enter fields like computer science and engineering, which are some of the fastest growing and highest paying.

"It goes a long way to showing it's not the students or the home, but the classroom teacher's behavior that explains part of the differences over time between boys and girls," said Victor Lavy, an economist at University of Warwick in England and a co-author of the paper.