There has been little to no increase for women on boards in corporate America, which is no longer only a gender equality issue. Diversity in the boardroom is also top of mind for investors who identify a correlation between greater gender diversity with better long-term financial performance and corporate governance.

Women held only 16.9 percent of board seats in 2013 — no change from last year (16.6 percent), according to the report, “2013 Catalyst Census: Fortune 500 Women Board Directors.”

In both 2012 and 2013, less than one-fifth of companies had 25 percent or more women directors, while one-tenth had no women serving on their boards. In addition, less than one-quarter of companies had three or more women directors serving together in both 2012 and 2013.

When it comes to diversity, the numbers are even less encouraging: Women of color held 3.2 percent of board seats, essentially the same as last year (3.3 percent), according to Catalyst. In both 2012 and 2013, more than two-thirds of companies had no women of color directors.

A group called the Thirty Percent Coalition is hoping to change these statistics by pushing companies to ensure their boards are composed of at least 30 percent women by 2015.

Last year, the Coalition contacted all companies in the Fortune 500 that had no women on their boards, and this month, the group sent letters to all 127 companies on the Russell 1000 that have no women on their boards, urging them to hire more.

In the letter, the signatories cite studies demonstrating a correlation between greater gender diversity among corporate boards and management, good corporate governance and long-term financial performance.

“What's new today is that substantial research underscores the correlation between gender diversity, good governance and positive long-term corporate performance. We are urging the business community to take this step not just because it's the right thing to do, but because it's the smart thing to do,” Thirty Percent Coalition Executive Director Charlotte Laurent-Ottomane said in a statement.

Diversity in the boardroom is one of the bigger issues that shareholders are starting to consider in the 2014 proxy season. Among U.S. and Canadian companies, there were 11 shareholder proposals calling for board diversity voted on during the 2011 proxy season, three in 2012 and nine in 2013, according to governance research firm GMI Ratings.

For more stories on board-related issues, check out InsideCounsel's coverage below: