"Part of the pay gap can be easily explained away. Women are more likely to leave the work force to care for children, for example, so they end up with fewer years of experience. Men also tend to work in higher-paying occupations and industries.
'But what you find is that when you pull out all of those factors, you still have about 40 percent of the wage gap — or 9.2 cents — unexplained,' said Ariane Hegewisch, a study director at the institute.
Academic research on gender and negotiation suggests that part of the unexplained gap may be tied, at least in part, to the negotiating process itself. It may be that some women have lower pay expectations. Men, on the other hand, have been found to be more likely to negotiate higher starting salaries."