While women are the sole or primary breadwinners in 40 percent of American families, according to a survey released this week by the Pew Research Center, women still make less on average than men do — about 77 cents for every dollar.
The debate over the gender wage gap has increased in recent months thanks partly to the best-selling book, "Lean In," by Facebook Chief Operating Officer Sheryl Sandberg, and President Barack Obama's commitment during the State of the Union speech to reduce the wage disparity between men and women. Obama advocates passing the Paycheck Fairness Act, legislation which makes it easier to prove gender discrimination in the workplace and toughens penalties for those who are convicted.
But the legislation is controversial. It has been introduced in the U.S. House of Representatives and Senate twice, failing to pass both times. Part of the controversy is that there is no consensus about why the gender wage gap exists.
Opponents of the Paycheck Fairness Act argue that the gap exists because women make different choices when it comes to work and family than men. But supporters of the bill say the wage gap is evidence of persistent discrimination of women in the workplace.
Skeptics of equal pay legislation argue that the census numbers are misleading.
"The census number compares the average earnings of all men with the average earnings of all women," said economist Stephanie Thomas of the Philadelphia-based consulting company Thomas Econometrics. "Those broad averages do result in the 23 cents per hour difference we've all heard of, but the statistic is not actually making an apples-to-apples comparison of men and women because it doesn't control for differences in education, experience or hours worked."
Thomas suggests pay difference is a reflection of women's unique preferences, not discrimination. For example, women tend to gravitate to more stable but lower-paying professions such as nursing and teaching, while men are more likely to seek out high-risk, high-reward opportunities.
"This creates a situation where women are concentrated in occupations that have lower earnings in general. When this happens on a large scale, then differences in pay between genders can result — and this happens without a difference in pay for men and women who are in the same job," Thomas said.
Women are also more likely than men to work part time, which means their total compensation will be lower, even if their employer pays the same wage rates for full- and part-time workers, said Thomas. The differences in total compensation aren't a result of discrimination, but again just reflect that women are choosing to work less.
Uncertainty about the veracity of the gender pay gap isn't the only criticism of the Paycheck Fairness Act. Some experts suggest the legislation would encourage a surge of unfounded class-action lawsuits. Small businesses would also find the new requirements particularly cumbersome, according to labor and employment lawyer Jane McFetridge.
"An employer who pays a man more than a woman for the same job might have to show that it's a 'business necessity,' " she said. "Do we want the government deciding what is business necessity? Isn't that for the business owner to decide?"
Economists who study gender and wages acknowledge that the census numbers don't tell the full story. Women are more likely to work in lower-paying occupations and leave the workforce when they have children, for example, said Cornell University labor economist Francine Blau, who found that accounting for women's choices and differences in education and experience only explains 60 percent of the wage gap. Their apples-to-apples comparison still leaves women earning 91 cents for every dollar that their male peers earn.
Cornell economist Shelley Correll says that discrimination against women, and in particular mothers, may explain the persistent wage disparity. For example, in a lab study, Correll and her colleagues Stephen Benard and In Paik asked participants to evaluate application materials for candidates of the same gender who were equally qualified but differed on parental status. They found that women who described themselves as mothers were perceived as less qualified, less committed, less promotable, less worthy of salary increases and less likely to be recommended for hiring.
Recognizing the limitations of a lab study, Correll's group adapted the materials from their experiment to see if the trends they observed held up in the real world. Applying to more than 600 jobs with their lab-generated resumes, they found that prospective employers called mothers back about half as often as non-mothers. By contrast they found men who were described as fathers were not disadvantaged.
Correll notes that discrimination isn't the only explanation for why women make less than men. Female workers may be less productive than their male peers, but since productivity is extremely difficult to measure, this is a difficult hypothesis to test empirically.
The Pew survey released Wednesday underscores dramatic shifts in family structure over the last several decades, fueled partly by declining marriage rates and an increase in the number of births that occur out of wedlock. There is also a big gap between median income for married couples, who make about $80,000 annually, and single mothers, who bring in about $23,000 a year.
While 79 percent of Americans reject the idea that women should return to traditional roles, according to the Pew survey, only 21 percent said the trend of more mothers working outside the home who have young children is a positive development for society.
"This change is just another milestone in the dramatic transformation we have seen in family structure and family dynamics over the past 50 years or so," said Kim Parker, associate director with the Pew Social & Demographic Trends Project told CBS News. "Women's roles have changed, marriage rates have declined — the family looks a lot different than it used to. The rise of breadwinner moms highlights the fact that, not only are more mothers balancing work and family these days, but the economic contributions mothers are making to their households have grown immensely."