A key tenet of modern feminism is that women will have achieved equity only when they fill at least 50% of the positions once filled by men. In some fields, women have already surpassed that target—now comprising, for example, 50.7% of new American medical students, up from just 9% in 1965, and 80% of veterinary students. But the needle has hardly moved in many STEM fields—such as the physical sciences, technology, engineering and math, in which barely 20% of the students are female.

A new study suggests some surprising reasons...