General Motors CEO and Chairman Mary Barra at the North American International Auto Show in Detroit in 2014. (AP Photo/Carlos Osorio)

The biggest news out of General Motors Monday may have been its agreement to invest $500 million in Lyft, the ride-sharing competitor to Uber, signaling a new era of competition in the auto industry. But Chief Executive Mary Barra had her own big news Monday, adding the title of chairman to her already massive job leading the country's largest automaker.

Barra, who became the very visible face of the auto giant as it navigated through its ignition switch crisis, will become only the 12th current female CEO in the S&P 500 index to also hold the title of chairman, a dual position many argue brings an executive unparalleled influence and power. If the 21 female CEOs in the S&P 500--just 4.2 percent, according to the research organization Catalyst--are a rarefied bunch, those who hold both titles are even rarer.

What's interesting, however, is that while the sample is obviously quite low, it's relatively more common for female CEOs to snag both jobs than for CEOs overall. Fifty percent of all CEOs in the S&P 500 index hold both positions, according to ISS Governance, a percentage that has been declining for years as investors push for companies to separate the two roles. (In 2010, for instance, 62 percent of companies combined them; in 2005 that figure was at 70 percent.)

But a slightly higher percentage of the 21 female CEOs--57 percent--at that level also wear the chairman's hat. That's despite the fact that women make up just 19 percent of all director seats at S&P 500 companies and just 16 percent at S&P 1500 companies, according to a new report from the Government Accountability Office, which found that it will take more than 40 years at the current pace for women to reach gender parity on corporate boards.

In addition to Barra, CEOs such as Lockheed Martin's Marillyn Hewson, PepsiCo's Indra Nooyi and IBM's Ginni Rometty are all members of this exclusive club. So are General Dynamics' Phebe Novakovic, Xerox's Ursula Burns and Duke Energy's Lynn Good. (The figure will dip slightly at the end of the month, when TJX Corp.'s Carol Meyrowitz plans to step down as CEO but remain as chairman.)

At numbers this small, it's hard to know whether the percentage of women holding both roles has any explainable significance. Is it because women are so often put into tough spots that holding both titles would let them work more efficiently? Or is it due to the type of companies that female CEOs end up running?

It's hard to tell, said Charles Elson, the director of the John L. Weinberg Center for Corporate Governance at the University of Delaware. "If you told me the number was 90 percent, I'd say it was significant," he says. "You could generalize and say maybe women become [chairman-CEOs] at more turnarounds, but it's hard to explain if it's a deviation at such a smaller scale."

Elson said the bigger question is why GM would choose to recombine the roles at a time when the practice of having a shared CEO-chairman is declining, and especially at a company whose history includes bankruptcy, government ownership, and the ignition switch crisis. "I've always believed a good bang-up CEO would want to have [the roles] split," he said. They'd "want a chairman to keep them from their own mistakes," he said.

Defenders of combining the two top jobs say that a lead independent director can serve as that monitor, too. Tim Solso, GM's outgoing chairman, is taking that very role as Barra ascends to her new post. In a statement announcing the change, Solso noted Barra's performance and said it was the right time for her to assume the chairman's role. "At a time of unprecedented industry change, the board concluded it is in the best interests of the company to combine the roles of chair and CEO in order to drive the most efficient execution of our plan and vision for the future," he said in the statement.

Whatever GM's reasons for the switch, and whichever way governance practices may be trending, one thing is clear: It's notable to see one domain where female leaders are keeping up with their male peers. Even those generally against the idea of combining the jobs see the upside in the numbers. "The fact that female CEOs are in the same position as their male counterparts is a good thing," Elson said. "It shows that people are viewing [them] as a CEO, not as a gendered CEO."

Read also:

Women still hold only 19 percent of U.S. board seats. What could change that?

These are the 12 major companies that still don’t have women on their boards

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