A lack of gender diversity in the boardroom is a problem not just for women in corporate America, but for investors, too. 

Seven companies in the S&P 500 have no female representation on their boards, according to data from BoardEx, a business intelligence service owned by TheStreet. Moreover, for many of them, that's been the case for at least a decade.

For women working at these companies who want to advance, it sends a bad signal that there isn't consideration for them at the top. For investors, it's worse: These companies tend to underperform the market.

"Most companies realize that you probably should have women and other diverse views in the boardroom," said Alan Johnson, managing director of New York-based compensation consulting firm Johnson Associates. "I don't think people want to be an outlier. They don't want to make the list of seven."

The seven companies identified by TheStreet as having no women on the board are Discovery Communications (DISCA) - Get ReportGarmin (GRMN) - Get ReportDiamond Offshore Drilling (DO) - Get ReportLinear Technology  (LLTC) Qorvo (QRVO) - Get ReportConcho Resources (CXO) - Get Report and Dentsply Sirona (XRAY) - Get Report .

Brande Stellings, vice president of corporate board services at nonprofit women's business organization Catalyst, was more cutting in her assessment. "In this day and age, in the 21st century, it is no longer acceptable not to have a woman on your board."

Over the past decade, women have made progress in this area. Per BoardEx, women comprised just 13% of S&P 500 boards 10 years ago; today, that number is up to 20%.

Research from 2020WOB, a national campaign that advocates for women on corporate boards, indicates progress is being made as well, though smaller companies especially are still lagging behind. Since 2011, Fortune 100 companies have added an average of 2.7 women directors per board. Smaller companies -- defined as those in the Fortune 501-1000 -- have averaged adding 1.7 women.

"We are gradually making progress," added Jessica Milli, economist and study director at the Institute for Women's Policy Research in Washington, D.C.

But a lack of gender diversity isn't just an issue women who aspire to leadership positions should care about -- it is something investors should pay attention to as well.

"There's a very strong business case to show a link between having gender diversity in leadership and improved financial performance," said Stellings, pointing to a raft of research that suggests correlation (not causation) between the two.

A February 2016 study out of the Peterson Institute for International Economics found that the presence of women on corporate boards and in the C-suite may contribute to firm performance. Researchers also found that among profitable companies, a move from no women leaders to 30% representation is associated with a 15% increase in net revenue margin.

A 2013 study from Credit Suisse, "The CS Gender 3000," found that companies with greater board gender diversity show excess stock market returns adjusted for sector bias as well as higher valuations and payout ratios. Return on equity (ROE) improves as well -- when there is one woman in the boardroom, companies saw an average ROE of 14.1% since 2005, compared to 11.2% for boards with all men.

Research from McKinsey suggests that the most gender-diverse companies are 15% more likely to financially outperform their peers, and ethnically-diverse companies are 35% more likely to do so.

"When you have higher percentages of women directors, those companies perform better on a variety of different measures," said Milli. "It makes sense, just logically. When you have greater diversity on your corporate boards...you tend to get different opinions, and you can ultimately reach better decisions for the company when you have those different opinions taken into account."

Women directors may also be more attuned to consumer desires, given that women drive 70%-to-80% of all consumer purchasing.

Still, some of Wall Street's most powerful investors have yet to take note.

Bloomberg pointed out recently that a number of big-name activist investors have widely overlooked women for their board seat nominations in recent years. Carl Icahn, who has nominated 42 individuals to fill 94 board seats at public companies in the past five years, has not put forth the name of a single woman. In fact, of 174 board positions five activist funds have angled for, women were nominated just seven times.

If the research is out there showing boardroom diversity matters, why are some of the smartest investors so slow to adjust?

"When you look at these companies, you have to wonder if they can't find a more diverse leadership, what else might they be missing?" Stellings said.

There are numerous explanations as to why the pace of change is so slow.

Credit Suisse points to cultural biases, workplace-related biases and structural and policy issues. The Peterson Institute paper floats factors like company size, degree programs, male-to-female income ratios, social attitudes, family leave and openness to foreign investment as possible justifications.

Charles Elson, director of the John L. Weinberg Center for Corporate Governance and a professor at the University of Delaware, says that improving boardroom diversity is largely a question of time. "This is a problem that's going to be solved ultimately through demographics," he said, though he admitted businesses that don't cast broad sweeps for directors "don't get talent that is as good as you should."

Stellings said the problem of networks is among the hardest nuts to crack in expanding gender diversity in corporate boards -- becoming a director isn't exactly something you can fill out an application for. "Being appointed to a board is about networks and who you know," she said. "It's very much about the network and who's in your network."

Johnson offered a broader critique: creativity.

"People generally aren't very creative when they look for board members," he said, noting that not only do boards often lack women, but they are also missing racial and ethnic minorities and those of a younger age. "The thing about a board is that you don't need 10 people that are all the same."

And it isn't that change can't happen -- in many places, it already has. There are a number of companies and countries out there that are getting gender balance in the boardroom right (even without imposing quotas, which many agree have mixed results).

Gap Inc. will be honored at the 2016 Catalyst Awards Conference for its Women and Opportunity initiative, through which the apparel company increased women's representation on its board of directors to 36% from 10% from 2010 to 2015. Navient, Tegna and Michael Kors all boast boards comprised of at least 50% women.

The UK government launched an initiative in 2011 to reach 25% board representation for women among FTSE 100 companies on the London Stock Exchange. Its latest report, filed March 2015, indicated representation climbed to 23.5% from where it started at 12.5%.

"In this day and age, to say that you can't find a qualified woman board director just holds no water," said Stellings.