It’s no wonder qualified female director candidates are being hounded. The all-male board is entering endangered species territory, as investors ratchet up demands that companies diversify and the #MeToo movement prompts a reexamination of women’s place in the business hierarchy.

But not all corporate ears are attuned to the cries for more female directors. Case in point: Twelve companies on the 2018 Fortune 500 lack a single female board member.

To be clear, these all-male bastions are outliers, with the trend going in the right direction. Five years ago there were 42 Fortune 500 companies without women directors; 10 years ago, 69.

It’s natural to look for commonalities among the holdouts, but few jump out. Four are in the energy sector, two are financial companies, and two operate in the food and beverage world. Three are based in New York City, three in Texas, and the rest are scattered around the country. Their employee headcounts range from 89,000 (Icahn) to a modest 1,607 (INTL FCStone).

The companies do, however, share one thing: a dearth of women in other top jobs. Among the 12 firms, there are just three women among the executives identified in Securities and Exchange Commission filings.

The businesses themselves offer few explanations. Eleven of the 12 did not reply to Fortune’s requests for comment. Only CHS responded. A spokesperson said that the board of the agricultural cooperative is nominated and elected “exclusively by our farmer-owners.”

Fortune is, in a sense, calling out these companies by listing them here. But we are far from the first to blow the whistle. In March 2017 index-fund giant State Street Global Advisors re-upped the issue in dramatic fashion, planting its now famous Fearless Girl statue across from Wall Street’s Charging Bull as it demanded that companies add more women to their boards. Paula Loop, head of PwC’s Governance Insights Center, says that move was “a really big deal”—in part because of what happened next.

In August 2017 passive investor Vanguard cited gender diversity in its demand that U.S. companies improve their governance. BlackRock, the world’s largest money manager, issued a similar warning five months later, stating publicly for the first time that the companies it invests in should have at least two female directors. In the U.S., says Loop, such investor pressure is now assuming a similar role to the one played by quotas in countries like France, Germany, and Italy, where women must account for a federally mandated share of board seats.

According to the investors, diverse boards are key to business success. State Street and Vanguard cite studies that show companies with more women directors outperform rivals without them. BlackRock argues that diverse groups make better decisions, a stance backed by research that finds that nonhomogenous teams are more likely to reexamine facts and maintain objectivity.

Katherine Klein, a Wharton professor, says meta-analysis shows a “tiny but positive” association between board diversity and financial performance, but she takes issue with the burden such studies foist on women. We don’t ask men to prove that simply adding them to a company’s board will boost its performance, she notes.

In this case, seven of the companies underperformed the Fortune 500 in total returns over the past five years. (CHS is private, Plains GP went public in 2013, and the remaining three had better returns.)

For some, the issue also has an ethical dimension. When New York State Comptroller Thomas DiNapoli announced in March that the state’s common retirement fund would vote on directors based on their boards’ gender makeup, he called it “unconscionable” that some companies still have no female directors.

But the 488 Fortune 500 companies not mentioned here should not necessarily assume moral high ground. Firms on the 2018 ranking control a combined $21.6 trillion in market value in a nation that’s 51% female. Yet as of March 31, just three boards on this year’s list had reached gender parity or better, according to Equilar. Estée Lauder’s board is nearing that esteemed territory. With Tejada and Hyman, its female representation reached 47%.

This article originally appeared in the June 1, 2018 issue of Fortune.