Her Pay Gap Begins Right After Graduation

For one, realize that it's not your fault, Babcock said. "It's liberating that it's not some inherent piece of my personality that I do this. Those are the voices that have been in my head over the years."

In a widely cited study from 1979, first-, fourth-, seventh- and 10th-graders were given a set task, then asked to pay themselves based on how well they thought they did. There was no difference between the sexes in the evaluations, but researchers found that in every grade, girls paid themselves 30 percent to 78 percent less than boys did.

Babcock said women should use a "cooperative negotiation style" to get what they want.

For example, don't go to a manager and say, "I have another job offer and unless you match it, I'll leave." That approach would be seen as threatening from a woman, even if it could be accepted from a man, Babcock said. So instead, reframe it: "I have this other offer, but I'd like to find a way to stay here. Can you match it so I can stay?"

Babcock also suggests practice. It may take a while for a woman to get over what she has been taught. So before negotiating, try some role-playing, she said. If you don't, you may ask for a raise and concede too fast or not negotiate at all. To prepare, sit with a colleague who knows the boss. Then go through different scenarios and ways to negotiate until you become comfortable with the process, she said. "We get most anxious when we don't know what to expect."

And once women know a little about what to expect, they may consider asking for what they want, as their male counterparts typically do.

Babcock conducted a study in 2002 that looked at starting salaries of students graduating from Carnegie Mellon University with master's degrees. The starting salaries of men were 7.6 percent higher, or almost $4,000 more, on average, than those of the women. It turned out, however, that only 7 percent of the female students had negotiated, but 57 percent of the men had asked for more money. The students who negotiated increased their starting salaries by 7.4 percent on average, or $4,053. That's almost exactly the difference between men's and women's average starting pay.

A lack of negotiating skills could be a part of the reason for the wage gap, said Hill of the AAUW report. Or it could go back to the person doing the listening. "Two workers who use the same kind of language could be perceived differently." In other words, a man and a woman might ask for the same thing in the same way, but get a different result.

In many cases, even women who finally ask for a raise may not get it because decision-makers aren't used to accepting negotiating behavior from women, Babcock said.

"Our society has a real double standard about what's acceptable for women to do and men to do," she said. "We're perfectly fine accepting negotiating behavior from men, but we react negatively when a woman does that. She knows she'll get a negative response or that we'll judge her, so she holds back."

At least two recent polls show that is likely happening. In another of Babcock's studies, she found 20 percent of women polled said they never negotiate at all. And in a recent study conducted by PINK magazine, a career publication for women, nearly half of 2,400 women surveyed didn't ask for a raise or promotion in the previous 12 months.

However, of those who did ask, 72 percent received one.

Knowing that, perhaps a few more women can gain some ground. And they just might set off enough change that today's young girls won't need to worry about a gap at all.