Corporations' boards of directors in the U.S. are still overwhelmingly dominated by men, and vague regulatory requirements on diversity haven't had much effect yet, leaving it to investors to push for change.

A recent report from Governance Metrics International calculates that the percentage of female directors, at just over 12%, has barely budged in the last three years. In fact, says Julie Gorte, senior vice president for sustainable investing at Pax World, a fund company with $2.7 billion in assets, the latest sizable increase in female representation on U.S. boards occurred a decade ago following the meltdown of companies such as Enron and WorldCom.

"There was a lot of board turnover, replacements and in some cases additions, after [the Sarbanes-Oxley Act] was passed, and a higher percentage [of seats] went to woman," Gorte says. "Otherwise, the progress has been glacial."