I remember laughing with my sister a few years ago when her daughter, then in elementary school, declared that “only women are doctors.” It was an understandable assumption: She had yet to meet a male physician. My sister is a doctor in a suburb of Washington, D.C., and her daughter’s pediatrician was also a woman. Initially my niece wanted to follow in their footsteps. Then she decided she wanted to be an astronaut. And then a couple of years...