Connie Glaser, an author and women's leadership expert, said at least part of the salary gap results from the fact that many women do not negotiate hard enough.
"Being liked is very important to women," Glaser said, and that can undermine the chance for a more lucrative salary.
Glaser cited a well-known passage in
Facebook COO Sheryl Sandberg's best-selling book, Lean In.
Facebook founder Mark Zuckerberg had recruited Sandberg and offered her a huge compensation package. Sandberg was thrilled, but when she proudly told her husband the amount, he said: "And you took it? You never accept the first offer made. You go back and tell him you want this amount."
Sandberg asked her husband: "But what if he thinks I'm greedy?"
Glaser said: "Even at that high level, women think about what people think of them. Guys don't have a concern with that. They think of negotiation as a sport. They like the back and forth. For women, it's like a root canal."
She says that's something women at all levels need to work on.
"One factor not addressed in the pay disparity is men's ability and willingness to negotiate," Glaser said. "I don't want to say it is women's fault ... but they will often accept what's on the table. So they're starting off at a step down."