Suppose you could eliminate the factors often blamed for the shortage of women in high-paying jobs. Suppose that promotions and raises did not depend on pleasing sexist male bosses or putting in long nights and weekends away from home. Would women make as much as men? Economists recently tried to find out in an experiment in Pittsburgh by paying men and women to add up five numbers in their heads. At first they worked individually, doing as many sums as they could in five minutes and receiving 50 cents for each correct answer. Then they competed in four-person tournaments, with...