Beth Ford, was was named CEO of Land OLakes last year, joined the sparse ranks of women who lead

The latestFortune500list is out, and theres a big reason to cheer: It includes the highest number of female CEOs ever, at 33.

While the overall share of women CEOs is distressingly low at just 6.6 percent of the total 500, female leaders made a big jump this year as compared to last, in which just 24 women helmedFortune500 companies.

The increase is attributed to several women who were named chief executive in the last 12 months, including Best BuysCorie Barryand Land OLakesBeth Ford. (Both women were named to ourMost Powerful Momslist this year.) A couple of new names on the list, Williams-Sonoma CEO Laura Alber and Advanced Micro Devices CEO Lisa Su, have held their jobs for years, but are joining the club now because their companies have surpassed theFortune500s revenue cut-off of $5.575 billion, the magazine reports.

As for why companies are suddenly willing to name more women CEOs, Lorraine Hariton, CEO of nonprofit Catalyst,toldFortuneits because boards have become more diverse. We are seeing women and minorities on boards ticking up, and boards have a lot to do with who becomes CEOs, she explained.

Research shows that when boards have female members, women are more likely to be appointed CEOs and are also more likely to have a lengthier tenure.

Unfortunately, diversity on boards doesnt yet seem to be helping women of color land the top job at the highest-grossing companiesonly a few women of color made this years list. Bed Bath & Beyonds Mary Winston is just the second black womaneverto serve as aFortune500 CEOand shes in an interim post.

However, theres hope those numbers will change, says Christy Glass, a professor at Utah State University who focuses on gender inequality and race and ethnicity in work and leadership, along withresearch co-authorAlison Cook.

Our research suggests that when women lead companies at the board rank and as CEOs, [theres] more attention [paid] to equality policies and practices, she toldFortune. So one added bonus of the growth of women directors is that CEOs place more emphasis on recruiting, retaining, and advancing people of color.

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