(Bloomberg) -- The boards of American companies with top-rated debt have at least one thing in common: they tend to have more women.

Women accounted for an average of 28% of directors at the five AAA rated companies in a study of 1,109 publicly traded North American companies, according to Moody’s Investors Service. That compares with 21% among the broader pool.

“Higher-rated companies tend to have higher levels of board independence, which may be a contributing factor,” Moody’s analysts including Brendan Sheehan and Ana Rayes wrote in a Sept. 11 report. “Additionally, many larger issuers have already began addressing gender diversity as the market has increasingly focused on this issue.” Larger companies are also more likely to have women on their boards.

Gender diversity tends to decline along the rating scale, Moody’s found. While women make up roughly a quarter of boards for companies rated Baa1 or higher, they occupied less than 5% of board seats for the two Ca rated issuers in the study.

Consulting firm McKinsey & Co. found that companies with the highest levels of diversity were 21% more likely to have above-average profitability than those with little diversity, using 2017 data. A Credit Suisse analysis of 3,400 companies worldwide in 2016 similarly determined that companies with more diverse management had higher returns on equity, dividends and market values, compared with less-diverse companies.

--With assistance from Jeff Green.

To contact the reporter on this story: Katherine Greifeld in New York at kgreifeld@bloomberg.net

To contact the editors responsible for this story: Benjamin Purvis at bpurvis@bloomberg.net, Emily Chasan, Nikolaj Gammeltoft

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