You make less money, but you are doing “good” work, so it’s worth it. You trade less money for a more flexible schedule, so you can take care of your family. You work for an organization that is struggling for money, so it doesn't bother you that your boss failed to find funding to get you a long-deserved raise. These are only a handful of the excuses that lead people in general, but particularly women, to accept lower salaries in socially-conscious professions.