Even the few women who've managed to advance to the C-suite don't get equal pay.

Last year, of the five best-paid executives at each of the Standard & Poor's 500 Index companies, 198 were women, or 8 percent of the total. Those high-achievers on average earned $5.3 million, 18 percent less than men, data compiled by Bloomberg show.

Four decades after substantial numbers of women began entering management ranks, only a handful have reached the echelons that earn the biggest compensation. Even after graduating from the same business schools, women tend to start out at lower salaries than men and many don't catch up later in their careers. Female executives say they can be less demanding than men when it comes to pay, partly out of fear of being labeled as overly aggressive and self-centered.