In the middle of a torturing pandemic, it’s the sort of milestone that raises some hope over gender diversity.

On Sept. 14, Linda Rendle became CEO of Clorox and the 38th woman now leading a Fortune 500 firm—the highest number ever in that elite corporate club. Just a few days earlier, another woman achieved a first when Citigroup announced that it will appoint Jane Fraser CEO next year, the first time a woman will lead a giant US bank.

Experts say these milestones are the result of five to 10 years of hard effort and may provide lessons to help the business world as it tries to shift to promoting more racial equality in leadership. But the pandemic itself poses risks to many of the strides women are making. With kids out of school physically, experts say younger women executives are being pulled into childcare roles more often than men, which is slowing or even cutting off career-rising prospects. “The ability to be the best of everything is a very heavy burden right now,” says Nina Boone, a Korn Ferry senior client partner and leader in the firm’s North America Diversity and Inclusion practice.

As it stands, the new milestone does not impress those who believe much more needs to happen. “It’s still less 10%,” says Jane Stevenson, global leader for CEO Succession at Korn Ferry. “We need this to become systemic and not just fortuitous.” For her part, Evelyn Orr, vice president and chief operating officer of the Korn Ferry Institute, worries that any progress can be reversed easily. “Before the pandemic, it still felt like an anomaly, and that a women candidate may have been a token.”

Indeed, recent history shows how fluid the numbers can be. In just the last six years, the number of women leading Fortune 500 firms has gone from 24 up 32, then back to 24 in 2017, and now up to 38.

Experts attribute the recent surge to an influx of women getting experience running large businesses below the CEO level. Successfully handling those roles has given more boards confidence that the female executives can become successful CEOs, Stevenson says. That’s what happened with executives such as Fraser, who has run Citigroup’s consumer banking business, or Corie Berry at Best Buy, who was the retailer’s chief strategic growth officer and president of its Geek Squad service business before she was named company CEO last year. It’s these under-the-radar promotions that have to continue in order for more women to become CEOs in the future.

At the same time, there is an increasing number of younger women who see becoming CEO as a viable career aspiration. That’s a big—and recent—change. In Korn Ferry’s 2018 study, Women CEOs Speak, 65% of the female CEOs said that they didn’t think becoming a CEO was a career option until they were actually approached to become one. The combination of more boards having confidence and younger women seeing themselves as CEO one day makes some experts hopeful that the recent surge has some staying power.

Still, women executives face a significant hurdle: unconscious bias, an obstacle that won’t disappear with the pandemic. Women who aspire to a CEO post are often viewed as overly ambitious, a trait that would almost never be assigned to a man who had similar aspirations, Orr says. Often female executives can be viewed as too outspoken as well, another attribute that would almost never be assigned to a male counterpart. “There’s a bit of an eye of the needle that woman have to get through in terms of being selected,” Stevenson says.

Experts say that the best hope for more women CEOs in the future may lie with the current crop of female CEOs. The more women the current female CEOs tap to run major lines of business strengthens the CEO talent pool. That prospect isn’t unrealistic. Two-thirds of the female CEOs in the Women CEOs Speak study were primarily purpose driven and wanted to change organizational cultures. “The agility women display daily just to juggle and balance the extreme demands in their lives has enabled their leadership duality to both perform and transform as today’s uncharted leadership agenda requires,” Stevenson says.

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