The #metoo movement is catching up with #oldboysclubs.

Institutional Shareholder Services, a top adviser to investors on corporate elections, is discussing whether to recommend "no" votes against directors of companies without any women on their boards.

Marc Goldstein, head of U.S. research for Rockville, Maryland-based ISS, confirmed in an interview that the internal deliberations are ongoing. A final decision would be made by November whether to adopt the new policy for annual shareholder meetings starting in 2019, he said.

"We have not historically used gender diversity as a factor for vote recommendations," Goldstein said. "We are considering whether it might be time to start doing that."

A growing number of investors and corporate-governance experts say an absence of women on boards can be a detriment to shareholder returns. Last year, State Street Corp. (STT) - Get Report , a big money manager, voted against the reelection of directors at 400 companies that failed to take steps to add women to their boards, the company said in a press release.

Under the glare of pressure, publicly-traded companies including the big U.S. banks Wells Fargo & Co. (WFC) - Get Report , Morgan Stanley (MS) - Get Report and Goldman Sachs Group Inc. (GS) - Get Report have added more female directors. Within the Standard & Poor's 500 Index of large U.S. stocks, the average share of women on boards is currently about 25%.

The push has gained momentum this year following high-profile sexual-harassment scandals at companies including CBS Corp. (CBS) - Get Report  and Wynn Resorts Ltd. (WYNN) - Get Report

Glass, Lewis & Co., another big shareholder adviser, announced late last year that, starting in 2019, it will "generally recommend" voting against the chairman of a board's nominating committee when there are no female directors, according to a spokeswoman for that firm. Nominating committees, for boards that have them, are tasked with identifying and recruiting new candidates.     

Last month, lawmakers in California passed a bill that would require companies based in the state to have at least one woman on their boards by the end of next year.  

ISS released a survey this week showing that more than 80% of investors consider it "problematic" for companies to have zero women on their boards of directors. That's up from 69% in a similar survey conducted in 2017. 

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The investors indicated that the lack of women on the board "may indicate problems in the board recruitment process," according to the firm.

Among firms in the Russell 3000 Index, which includes most public companies on major U.S. stock exchanges, 485, or 17%, had all-male boards in the second quarter, according to Equilar, a research firm that gathers data on executives and boards. That's down from 21% last year and 37% in 2012, according to Goldstein. 

"The events of the past year are not irrelevant, in terms of the attention paid to sexual harassment in many areas of society, including in the corporate world," Goldstein said. "I don't think too many boards or companies want to be perceived as so clueless or out of touch that they have no women on the board."

For companies with no female directors, ISS could recommend that shareholders vote against the chairman of a board's nominating committee, Goldstein said. It's undecided whether any new policy would apply only to large stocks, such as those in the Standard & Poor's 500 Index, or to a broader universe of companies, such as those in the Russell 3000, he said.

There's also a chance that ISS might allow companies until 2019 to add female directors before recommending any "no" votes, he said.

"They'd have to have their heads buried pretty deeply in the sand to not be aware that this is a concern for investors," Goldstein said. "But we don't want to put companies in a position where they have to just take the first woman they find."  

The thesis that a higher percentage of female directors can improve corporate performance is supported by an organization called the 30% Club, where the CEOs of the money managers BlackRock Inc. (BLK) - Get Report , State Street and Vanguard Group are members.

Diverse perspectives help to foster healthier debates within boardrooms, the theory goes, resulting in more effective strategic decision-making and potentially higher long-term growth.

And the difference becomes clear when a board has at least three women, ensuring that their contributions will be fruitful, proponents say. The dynamic is partly explained by the reality that when there are least three women on the board, they're more likely to disagree with each other, forcing male directors to recognize that the women's individual perspectives aren't solely gender-based.

Companies with three or more female board directors in at least four of the previous five years reported a 46% higher return on equity, a key measure of profitability, than those with lower representation, according to a 2011 study by Catalyst, a nonprofit organization that advocates for boardroom diversity. In 2016, the investment-research and index provider MSCI Inc. reported that three female directors may constitute a critical mass allowing women to contribute more equally to group decision making.

"The rationale is that a diverse board is less prone to groupthink," Goldstein said.