"Does that reflect an injustice? Many say no, arguing that women earn less because they take time off to care for children or elderly parents (and thus have less seniority) or accept lower pay in return for more flexible working arrangements. Women also tend to go into lower-paying lines of work, shunning higher-paying technical fields. It's easy to caricature this view (dirndls versus Visigoths, etc), but there may be some truth in it. Some research suggests that when women behave as men do--not having babies, mainly--the income gap largely disappears. If so (I won't claim the matter has been definitively settled), the question facing women is a stark one: What do you want, kids or cash?"

[1] http://www.straightdope.com/columns/read/2411/are-women-paid...