Setting aside the question whether insurance should cover the Pill — an issue attracting some attention of late — that form of contraception has made a tremendous difference in women's career arcs and earnings, a new study finds. In fact, the Pill, by itself, was responsible for 10% of the narrowing of the male-female pay gap in the 1980s, and nearly a third of the narrowing in the 1990s.

The study, by Martha J. Bailey and Brad Hershbein, of the University of Michigan, and Amalia R. Miller, of the University of Virginia, took...