The AAUW produced a study in 2012 that compared the hiring experiences of men and women one year following college graduation. The gender pay discrepancy was there, all right. But how much was due to the known factors mentioned above?

Working through the numbers, the researchers concluded that all but 7 percent of the gap could be explained by the known factors. That final 7 percent? Pretty much pure bias against women, Hill said.

"What we see is implicit or unconscious bias that is influencing people that they are not yet fully aware of," she said.

There's an inherent bias against hiring and promoting women that emerges most clearly in the "male jobs" realm, which includes tech, medical and research positions among others.

Hill cited an academic study that involved sending the same resume, one allegedly from a man and one from a woman, to men and women with hiring authority in a STEM research facility.

"Both men and women rated the man higher — in pay, working environment and willingness to mentor them," she said. "This demonstrated what we've long known — that men and women still harbor implicit biases against women in the workplace."

Hill said the days of "explicit" discrimination against women are largely behind us. Now the task is getting folks to acknowledge and work on the implicit discrimination, to address that 7 percent-plus that remains "unexplained."

What would she recommend as a starting point to address this gap? She would make critical adjustments in behavior and education early in a child's life.

"Girls and boys would benefit from special skills learning — girls tend to lag behind boys, but you can learn it," she says. "And I would encourage people to have boys and girls play together more. Sometimes these stereotypes are in place when they're quite young. If they could play together, they would get to know each other in a full and real way."