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.

“I was always focused on negotiating for my team but never as good at negotiating for myself,” said Dawn Lepore, the former chief executive officer at Drugstore.com and former vice chairman of Charles Schwab Corp. who’s now a director at AOL Inc. and TJX Cos. “When I think about the way I negotiated my package when I went to Drugstore.com, I thought ‘this is a great opportunity and rather than demand a lot upfront, I’m going to do a great job and then I’ll get rewarded.’ I didn’t want to be greedy, or viewed as out for myself. Maybe women are more collaborative.”

Topping the compensation ranking of female executives, compiled from the five highest-paid executives listed in the proxies of each S&P 500 company for the year 2012 or its fiscal equivalent, were Oracle Corp. Chief Financial Officer Safra Catz with $51.7 million and Yahoo! Inc.’s Marissa Mayer with $36.6 million. While both got higher compensation than some peers, their examples are more the exception than the rule.

Paid Less

Facebook Inc.’s chief operating officer, Sheryl Sandberg, who advises women in her best-selling book “Lean In” not to squelch their ambitions, notes that women’s lack of sponsors in their workplaces keeps them from asking for both stretch assignments and raises.

Women are such a rarity in top jobs –- about 20 S&P 500 CEOs -- that they may not want to stand out more by being tough on pay, according to Pat Cook, president of boutique executive search firm Cook & Co.

“Women tend to start out their careers getting paid less, and that gap often never gets made up, even at the most senior levels,” said Cook, whose firm is based in Bronxville, New York. “Plus women settle more frequently than men for what they’re offered.”

CEO Scarcity

The scarcity of female CEOs explains in part the S&P 500 gender gap. The small pool brings the average compensation down among women in the C-suite because chief executives tend to command the highest pay, said Ilene Lang, president of Catalyst, a New York-based research and advocacy group for women executives.

Chief executives are lagging behind their male counterparts in certain industries. Take Denise Morrison, CEO of Camden, New Jersey-based Campbell Soup Co.: She got total compensation of $8.76 million last year, 24 percent less than the average earned by CEOs at food, beverage and tobacco companies, according to data compiled by Bloomberg. Mylan Inc.’s Heather Bresch was paid $9.96 million, 33 percent less than the average chief of a pharmaceutical, biotechnology and life sciences company.

‘Large Gap’

“It is unfortunate that in many instances there is still a large pay gap between men and women in corporate executive positions,” said Maggie Wilderotter, 58, CEO of Frontier Communications Corp. since 2004 and a director at Procter & Gamble Co. and Xerox Corp. “I am hopeful that the use of outside compensation consultants by the compensation committees of public boards will make the gaps more evident and get inequalities addressed.”

Many factors contribute to pay levels, including performance, age and experience. Both Morrison, 59, and Bresch, 44, are relatively new in their CEO jobs, since August 2011 and January 2012, respectively.