It continues to amaze me how few people understand the concept of comparable worth or pay equity, and insist on referring to the free market as though a totally free market based upon supply and demand actually exists.
The market is an historic development rather than an expression of natural law. The theory underlying comparable worth begins with the observation that the sex/gender system as it has historically been incorporated into the economy has created a context in which women's work, whatever its content, is systematically devalued. Supporters of comparable worth argue that compensation should be independent of the social characteristics of the workers and based instead upon factors such as education, skill, and level of responsibility.
Information on the wage gap is plentiful. The Institute for Social Research located at the University of Michigan (which has the world's largest repository of computer-readable social science data and an international reputation of excellence) recently concluded an extensive examination of this topic. Multiple explanations for wage disparity were considered, including age, sex, race, education, workers' attitudes, occupation, involuntary unemployment, illness, relocation, and family composition changes. The conclusion of the largest, most impressive study ever done in this area was that the sex-based wage gap reflects institutionalized discrimination against women in the paid labor force.