Women have made gains in breaking through the glass ceiling in the past year, new research finds. Despite those gains, however, women still occupy only 15.6 percent of boardroom seats in the largest public companies in America.

Overall, growth equaled 1 percentage point in the past year among Fortune 1000 companies, but was larger among companies listed between 501 and 1000 by Fortune. Women made up 13.6 percent of board members at those companies, up 1.2 percentage points in the past year.

Among companies listed in the Fortune 1000, the number of companies with 20 percent or more women on boards also grew 13 percent, up to 308 from 273. Additionally, the number of companies with no women on their boards fell 14 percent, from 177 in 2011 to 152 in 2012.

"The campaign to put more women on corporate boards is working," said Stephanie Sonnabend, president of Sonesta Collection Hotels Resorts Suites and co-founder and chairwoman of 2020 Women on Boards, a national campaign to increase the percentage of women on public company boards to 20 percent by 2020. "After years of no improvement, this year Fortune 1000 companies added 74 women. Companies of all sizes are getting the message that it's time to put women on their boards."

On average, there are still fewer than two women on the boards of companies listed on the Fortune 101 through 1000. However, companies listed on the Fortune 100 have more than two women on their boards.

"We will continue with our education and advocacy efforts to educate companies that board diversity is a necessary and profitable business practice today," said Malli Gero, co-founder and executive director of 2020 Women on Boards, which conducted the research.

The research was a part of the 2020 Women on Boards Gender Diversity Index. The research was based on data from the Fortune 1000 list.

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