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The number of women CEOs in the Fortune 500 is at an all-time high — of 32

General Motors Chairman and CEO Mary Barra speaks about the financial outlook of the automaker in Detroit on Jan. 10. Barra is the highest-ranking female CEO on Fortune Magazine's newly released Fortune 500 list. (Paul Sancya/AP Photo)

The percentage of women running companies in the Fortune 500 is still solidly in the single digits, but the proportion is slowly growing, and just reached an all-time high.

On Wednesday, Fortune Magazine released its 2017 Fortune 500 list, which ranks major U.S. companies by their fiscal 2016 revenues, and 32 of the companies, or 6.4 percent, were run by female CEOs.

The magazine said that number was the highest proportion yet seen in the 63-year history of the list. Last year, it had just 21 women CEOs on the list, and for the 2014 fiscal year, there were 24.

The surprising role where women consistently earn more than men

Roughly a third of the women running companies on the list are new to the job: A startling 11 women were named CEO of companies in 2016 or early 2017 that are on this year's Fortune 500 list, including Vicki Hollub at Occidental Petroleum, Tricia Griffith at the insurer Progressive, Shira Goodman at Staples, Margo Georgiadis at Mattel and Michele Buck at Hershey. At Hertz Global Holdings, Kathryn Marinello was named to the top job late last year, as was Anna Manning at Reinsurance Group of America.

That's a significant jump from one year to the next, and comes at a time when more attention is being paid by investors to the value of diversity in the boardroom. Investors are pushing shareholder proposals that urge companies to add diversity among their top ranks, and financial companies are adding products that let investors bet on companies with more women at the top, as research demonstrates a link between diversity and better performance.

Only one woman was named CEO of a major company in North America in 2015

The Fortune 500 list, which includes private and public companies, has more female CEOs than the Standard & Poor's 500-stock index, which had 29 female CEOs as of April, according to a list compiled by the nonprofit firm Catalyst. Though the lists are different, the bumper crop of female CEOs named to run Fortune 500 companies last year is worth noting, given that in the year prior, only one woman was named CEO at an S&P 500 firm.

Women run seven of the country's very largest companies, the Fortune 100, including Mary Barra at General Motors, Ginni Rometty at IBM, Indra Nooyi at PepsiCo and Marillyn Hewson at Lockheed Martin.

That may be one reason executive compensation experts say, that the median female CEO, surprisingly, makes more than her male peer. While the explanation for why is unclear -- it could, of course, simply be due to better performance, or companies could pay a "diversity premium" for a female CEO -- the median female CEO made $13.1 million in 2016, compared with $11.4 million for the median male CEO.

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