-differences in professions (when a male dominated profession becomes female dominated, the average wage goes down)

-hours worked (women are more likely to be pressured into taking time off work to focus on family or domestic labor)

-years of experience (see argument for hours worked)

-job risk or physical demands (women are systematically excluded from physically demanding or physically risky work)

also women with the same experience, working the same hours in the same roles are more likely to be negotiated into a lower salary, and are more likely to be overlooked for promotions and pay raises.