The direction a company takes on diversity comes from its board room, where white men have dominated since the last century. When choosing a new CEO, board members tend to rely more often on people they know than on executives selected by recruiters who screen candidates from a wider field, said Trina Gordon, CEO of executive-search firm Boyden. About 80 percent of S&P 500 directors are men.

“Boards are still not very diverse, and if you don’t have diversity at the governance level, there’s not a lot of changes that are going to happen,” Gordon said.

Women, who make up about half of the U.S. work force, aren’t forecast to gain parity in the board room until 2032, according to a June estimate from executive recruiter Heidrick & Struggles.

Debra Crew’s promotion to succeed Susan Cameron as CEO at Reynolds American Inc. earlier this year was the first female-to-female handover in the S&P 500 in five years, according to data from recruiter Spencer Stuart. The distinction was short-lived because even before she took the job, British American Tobacco Plc said it would buy Reynolds. Now Crew reports to BAT CEO Nicandro Durante.

Prior to that transition, the last time a female CEO was replaced by another woman was in 2012, when Sheri McCoy succeeded Andrea Jung at Avon Products Inc. McCoy announced her resignation Thursday from Avon, which is no longer in the S&P 500. A successor hasn’t been named.

The third example in recent years was Xerox Corp. CEO Ursula Burns, who replaced Anne Mulcahy in 2009. Burns retired this year and was replaced by Jeff Jacobson, her former chief operating officer.

When female CEOs step down, it’s typically a man waiting in the wings. Among the 27 S&P 500 companies run by women, most chief operating officers or presidents are men. At PepsiCo Inc., Indra Nooyi appointed Ramon Laguarta as president last month, setting him up as one of her possible successors.

A big issue is that companies often prefer to make a safe choice for leadership, selecting an executive with a track record for running a company or a large unit, and those executives are still overwhelmingly male, said Gordon.

“The statistics are daunting,” she said. “There is a perpetuation of the status quo.”

Female chief executives are judged differently than men, another hurdle women face on the path to the C-suite, said Barbara Annis, founding partner of Gender Intelligence, a company that focuses on improving workplace gender relations.Research shows that women are judged on performance, while men are given opportunities based on their potential, she said. And when a woman is at the helm and a company performs poorly, frequently her gender is viewed as the reason.

“We attribute results to one’s gender when it comes to a woman, and we don’t do that with men,” Annis said. “So often when that happens, we end up picking a man as the successor.”

The different expectations also extend to how activists approach female CEOs, according to a 2016 report from a researcher at Arizona State University. For every 10 shareholder proposals at a company led by a man, there were 15 for a company with a female CEO. Those women had a 27 percent chance of being targeted by activists, while the risk to men was almost zero, according to the study, which examined shareholder proposals at Fortune 100 companies from 2003 to 2013.

Even as the number of women running the biggest companies falls to 25 from 27 this year — thanks to the departure of Crew and Rosenfeld — it remains a relatively new phenomenon to have so many at one time, Stellings from Catalyst said. Between 1972 and 2000, there were never more than four female CEOs serving at one time, she said.

“You could easily fit all of them, in the history of our time, into one room,” Stellings said.