FORTUNE -- While the CEO appointment of Mary Barra at General Motors (GM) is historic (no other woman on earth has ever led a business so big), here's another interesting fact that went largely unnoticed: When Barra joins the GM board on January 15, GM will become the rare Fortune 500 company with almost as many women directors as men.
Once Barra, who turns 52 later this month, replaces retiring CEO Dan Akerson in the boardroom, GM will have five female directors on its 12-member board. That's 42% of the lineup.
GM, of all companies! But the new gender balance makes sense since women consumers, according to GM, make more than half of the buying decisions in the auto industry.
The rarity of such board diversity is proven in a study released this week by Catalyst: Only four Fortune 500 companies--Estee Lauder (EL), Avon Products (AVP), Procter & Gamble (PG), and ad giant Interpublic Group (IPG)--have boards with more than 40% female directors.
While these companies are mixed in overall performance (Estee Lauder, with a board that's 46.7% female, is on fire), research shows that companies with diverse director slates generally deliver superior returns to shareholders.
Yet progress is, as GM's Akerson said publicly last month, "glacial" and "unacceptable." As I wrote in Fortune's recent Most Powerful Women issue, the corporate boardroom appears to be "the last glass ceiling" where only 17% of Fortune 500 directors are female. Catalyst reports that 50 Fortune 500 companies still have no female directors.
There is one sign of hope, though. "Demand for digital expertise is bringing more women onto boards," says Jana Rich, who heads the consumer digital and media practice at search firm Russell Reynolds. Last week in San Francisco, Rich hosted a panel (part of TEDWomen) called "Digital women on Fortune 500 Company Boards." The directors on her panel were Sue Decker, Dawn Lepore and Ali Rosenthal.
Decker, a former president of Yahoo (YHOO), is now on the boards of Berkshire Hathaway (BRKA), Costco (COST), Intel (INTC), and LegalZoom (LGZ). The dearth of women on boards is so troubling to her that she's altered her view against quotas. "It's not going to happen naturally," Decker says. "Until you're forced to be creative, I don't know that equal representation will happen fast enough, given how few positions open up each year."
In fact, low board turnover is stunting progress. Rich cited a study showing that only 151 board seats at Fortune 500 companies turned over last year. This was a 10-year low.
Decker doesn't want the U.S. government legislating targets, as some European countries have done. She prefers Fortune 500 boards setting and complying with their own individual goals. Though she knows there would be fallout: "If boards do impose quotas, they'll need to tolerate making mistakes from time to time because it will force them to draw from pools of talent that may not have as much experience in C-suites."
"I have very mixed feelings," Lepore responded to Decker's proposal. A decade ago, Lepore was a CEO struggling to revive drugstore.com, which is now part of Walgreen (WAG), and she needed the best board members, bar none. "I agree conceptually in quotas, but I have concerns practically," she said.
Lepore, a former vice chairman in charge of technology at Charles Schawb (SCHW), shows how conducive a tech background can be to getting on great boards. Once a director of Walmart (WMT), eBay (EBAY) and the New York Times (NYT), she's now on the boards of AOL (AOL), retailer TJX (TJX) and RealNetworks (RNWK).
Meanwhile, demand for digital directors is opening up opportunities for younger women. Starbucks (SBUX) has Clara Shih, the 31-year-old founder and CEO of Hearsay Social. Walmart has Marissa Mayer, 38, who got recruited before she moved from Google (GOOG) to be CEO of Yahoo. Facebook (FB) VP Carolyn Everson, 41, is on the Hertz (HTZ) board. Ali Rosenthal, ex-Facebook (FB) and now COO at a startup called MessageMe, joined the board of AutoNation (AN) in 2011--when she was 34 years old and, she admits, she did not know much about cars. But Rosenthal, now 37, certainly knew social media.
On Monday, Kim Jabal, CFO of social-networking startup Path, joined the board of Fedex (FDX). A 45-year-old engineer with a Harvard MBA, she's joining Fedex's Information Technology Oversight Committee.
Meanwhile, the tech industry fares worst of all in terms of diversifying its boards. Ernst & Young reports that 39% of tech companies have no female directors. Twitter's (TWTR) appointment last week of Marjorie Scardino, the digital-savvy ex-CEO of Pearson, was one small step for women on boards, and calmed some of the critics.
But it will take more women like Barra, armed with an engineering degree and a Stanford MBA plus lots of ambition, to take corporate America where it needs to be.
For more on the topic, read my Postcard on "How women can break into the boardroom."
General Motors' designated CEO will run the largest woman-led Fortune 500 company.
Mary Barra, the newly named CEO of General Motors (GM), will become the 23rd female CEO of a Fortune 500 company.
And based on revenue--which is what the Fortune 500 is all about--she tops the rankings of women in charge of America's largest companies.
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by Patricia Sellers
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by Jessica Shambora
When Ellen Kullman stepped into the CEO role at DuPont (DD) this month, she became the 13th female Fortune 500 CEO. That's a milestone. But Kullman's ascension passed with little fanfare. She addressed employees in a year-end video on DuPont's intranet site but has yet to address them as a group or to send out a company-wide email since the start of the year.
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