Business environment can also count. "Unionization tends to flatten differences" in pay, says Catherine Hill, research director at the American Association of University Women. That means that pay gaps may be wider in "right-to-work" states, where laws stop union and employer agreements that would require workers to pay union dues. Meanwhile, non-right-to-work states, concentrated in the Northeast, around the Great Lakes, and on the West Coast, may have smaller wage gaps.
All of these, as well as broader factors--women taking time out from their careers to take care of children, for example--can contribute to the broader wage gap in the U.S. With all these forces at play, it's impossible to parse out exactly why one particular place might have a smaller wage gap than another, especially with one other unquantifiable factor at work:
"I have little doubt that there are some parts of the country where there's more zealous discrimination than others," says Burtless, "but you can't interpret difference as being a gauge of how much discrimination there is."
Danielle Kurtzleben is a business and economics reporter for U.S. News & World Report. Connect with her on Twitter at @titonka or via E-mail at firstname.lastname@example.org.