Women get paid less than men in almost all jobs, but when women in low-wage jobs need to take time off work to care for children, they are at an even greater disadvantage.

If all employees got paid the same hourly amount (assuming they’re equally productive on the job), it would go a long way toward closing the gender pay gap, according to Claudia Goldin, a Harvard economist who has analyzed income data across occupations, including a new set of unpublished data on hourly workers that she prepared for the White House Summit on Working Families in June.

Instead, she has found, people in professions like law and finance get paid disproportionately more when they work extra-long hours. At the other end of the spectrum, people in low-wage jobs do not benefit much from working more, but get paid disproportionately less per hour when they work fewer than 40 hours a week. The penalty is similar for men and women — but ends up hurting women more, because they are far more likely to take breaks during their careers or need shorter or predictable hours to handle child care.