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By Jeff Cox, CNBC

Gender equality is still a long way from the executive suites of corporate America.

The nation's largest companies remain laggards in terms of getting women into CEO positions, according to a new study that shows S&P 500 company leadership remaining the domain of men, with almost no gains over the past nine years.

In fact, 2015 saw regression, with the total number of female CEOs dropping from 25 in 2014 to 21 as 2015 comes to a close, according to the study from Pavle Sabic, director of market development at S&P Capital IQ.

"Despite all of the attention placed on increasing the number of female executives at American companies, the needle on the gender gap has hardly moved," Sabic said in a narrative accompanying the study. "The gender gap at the CEO level of S&P 500 companies is not closing. Growth rate for new female CEOs is only one per every two years."

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The decline in female top executives comes even amid evidence that they generally outperform their male counterparts.

A study early this year from Bespoke Investment Group found that share performance at companies led by female CEOs had been topping male-run companies by 5 percentage points. In 2015 as a whole, female-led companies are averaging returns of 0.19 percent, compared to the S&P 500 index price decline of nearly 2 percent.

Since 2005, the total number of female CEOs has grown from 16 to 21, an average of just one every two years.

The U.S. has trailed its global competitors generally when it comes to gender equality. The most recent ratings from the World Economic Forum on the issue rank the U.S. 20th of 142 countries, with Iceland first and Germany, France and Canada all ranking higher.

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Tenure for female CEOs tends to be shorter, though, Sabic found. Female CEOs stay with their companies an average of four years, against six for the men.

"While four to six years is not largely significant in the context of a lifelong career, it may translate to a significant economic opportunity cost to women," Sabic wrote.

In terms of sectors, information technology has the most female CEOs with five, while energy, materials and telecom have none. Tech is the third-best performer of the S&P 500's 10 sectors this year, returning 4.7 percent, while materials ranks ninth, with a 9.6 percent decline, and energy is last with a 23.1 percent plunge. Telecom is fifth with a 0.6 percent decline.

The S&P 500 as a whole has 96 percent of companies run by men, down only slightly from the 97 percent in 2006.