On this day 50 years ago, President John F. Kennedy signed the Equal Pay Act in an effort to abolish wage discrimination based on gender. Half a century later, the Obama administration is pushing Congress to pass the Paycheck Fairness Act, designed to make wage differences more transparent.

Some dispute the frequently cited figure that women are paid 77 cents for every dollar a man earns. But even those who argue the gap is narrower agree it's most prominent when a woman enters her childbearing years.

'She's Just Going To Leave Me'

In 2010, an analytics firm called Reach Advisors crunched Census Bureau numbers and found something surprising: The median salary of single, childless women under the age of 30 was 8 percent higher than their male counterparts. That's largely because more women are going to college than men.

What made that number noteworthy is that it's the only group of women who have a pay advantage. In fact, different numbers from Reach Advisors show that that early advantage vaporizes later in women's lives — especially if they have children.

"Studies have shown for over a decade that what is really killing women economically is motherhood," says Joan Williams, professor at the University of California Hastings College of Law. She popularized the term "maternal wall," referring to discrimination against hiring or promoting mothers based on the assumption she will be less committed to her job.

"We run a hotline, so I hear about it most every week," Williams says.

The number of pregnancy and maternity discrimination charges filed with the Equal Employment Opportunity Commission has steadily increased since the late 1990s.

"I think what's interesting is that it's against the law, I assume everybody knows it, yet it still occurs. And it occurs in a very overt fashion," says David Lopez, the agency's general counsel. "That means that employers often leave trails of emails saying, in effect, 'We don't want her because she's a mother.' "

Catherine Hill is research director for the American Association of University Women. She says the pay advantage young women supposedly have disappears when you control for the level of education. Her research shows that, just a year out of college, women are at a 6.6 percent pay disadvantage. Hill says a big part of that is the anticipation of motherhood.

For example, she says, employers may ask themselves: Am I really going to spend the money to put this woman into a training program? She's just going to leave me, after all.

"And that kind of discriminatory behavior really boxes women in, so we all get penalized because people fear that women are going to leave the workplace," Hill says.

And she says the pay gap compounds over time, hurting women in their retirement years.

A First Step Or Step Back?

Not everyone agrees there's a link between the pay gap and discrimination.

Sabrina Schaeffer is executive director of the Independent Women's Forum, a conservative and libertarian women's advocacy group.

"I don't deny that there is a wage gap. The question for me is, how big is that gap, and what are the causes?" Schaeffer says.

Schaeffer attributes the so-called motherhood penalty in pay to choices women make to work fewer hours or take time off. Less experience adds up to less pay. And, she says, anti-discrimination laws paradoxically hurt working women.

"I absolutely think that we would be better off without the Equal Pay Act," Schaeffer says.

She says rules and regulations make it harder for employers to grant flexible work arrangements because they're required to track pay and hours worked. So women make the tradeoff to work less and get paid less — and that's not necessarily bad.

"We often talk as if men and women live in a vacuum, and as if what's good for my husband or what's good for me doesn't benefit the other person. At the end of the day, as I see it, our interests are tied. So I think that the idea of a motherhood penalty is a little bit outdated," Schaeffer says.

But equal pay activists say it is, in fact, the future. A recent Pew Research Center study showed 40 percent of moms are breadwinners — and that a majority of them are single.

Williams, the Hastings law professor, says the pay gap should not be framed as a matter of choice.

"Women choose to have babies; they don't choose the discrimination that goes along with it," she says.

Speaking 50 years ago, President Kennedy hailed the Equal Pay Act a "first step." He then asked Congress to fund tax breaks and day care centers so women could remain in the workforce even after they became mothers.

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