According to data complied by Bloomberg, last year, of the five best-paid executives at each of the Standard & Poor’s 500 Index companies, only 198 were women, or 8 percent of the total. They on average earned $5.3 million — 18 percent less than men.

Women tend to start out at lower salaries than men, and many don’t catch up later in their careers, even after graduating from the same business schools. The study notes 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.

Women are such a rarity in top jobs  that they may not want to stand out more by being tough on pay, said Pat Cook, president of boutique executive search firm Cook & Co.

M.J. Tocci, director of the Heinz Negotiation Academy for Women at Carnegie Mellon University, encourages women at all levels to push for equal pay. “I tell women who say they don’t care about money that they should still make what they’re worth — and they can always give their money away,” she said. “Women who accept less than what men get for the same jobs are lowering the bar for the women who come after them.”