In 2009, the U.S. DOL commissioned a study of the gender wage gap.  The report of that study is available at http://commons.wikimannia.org/images/Gender_Wage_Gap_Final_Report_2009.pdf. In summary:  more women work part time than men so they earn less; more women leave the workforce to take care of their families and when they return to work have less work experience to justify higher salaries; more women choose jobs with flexible schedules and lower wages; and more women choose jobs in low-wage industries such as child care.  Once the study adjusted for all these factors, the wage gap was not 30 or 40 percent, it was closer to 5 to 7%.  In addition, the national data available makes it difficult or impossible to compare apples to apples such as wages earned by men vs. women in the same job.  Bottom line, while there is likely still discrimination by some employers (which is illegal) most of the wage gap is explained by factors of education, experience, and worker choice.