Once upon a time, little girls went to school and were told that if they studied hard and got good grades, they would be able to pursue good careers and live in economic security ever after.

But a funny thing happened when those girls became women and entered the workplace: they were paid less than the men in the workplace — often a lot less. Once word started to spread that women were being paid less, a list of reasons for the difference in pay was developed. Women preferred lower paying jobs, like child care, teaching, nursing, and waitressing. Women weren’t as interested in higher paying jobs in science, technology, engineering and math. Women weren’t as good at negotiating pay as men were, so they didn’t ask for enough money. And women weren’t as committed to their careers, so they took time off for having children and they often left jobs to follow their husbands when their husbands got promotions that required them to move out of town.

Well now comes a study that suggests gender bias is actually a significant factor in the gender pay gap. Claire Cain Miller wrote in The New York Times on Friday about a new study from that indicates “when women enter fields in greater numbers, pay declines — for the very same jobs that more men were doing before.” The Times article cites a study three sociology professors: Paula England of New York University, Asaf Levanon, of the University of Haifa in Israel, and Paul Allison of the University of Pennsylvania. The article says:

A striking example is to be found in the field of recreation — working in parks or leading camps — which went from predominantly male to female from 1950 to 2000. Median hourly wages in this field declined 57 percentage points, accounting for the change in the value of the dollar, according to a complex formula used by Professor Levanon. The job of ticket agent also went from mainly male to female during this period, and wages dropped 43 percentage points.

Similar changes in pay were seen when women became designers, housekeepers and biologists, The Times article says. But when fields that traditionally had more women began to attract large numbers of men, the pay increased.

Responding to The Times article, Emma Cueto writes on the Bustle website: “Whatever else we can say about the wage gap, the chicken and the egg question of “Are women low paid because they work low-paying jobs or are jobs low-paying because women work them?” has officially been solved. Women are paid less because our work isn’t valued in the same way that men’s is. And that is sexist, no matter how you look at it.”