Just over 15 per cent of all director seats at publicly traded US companies were held by women as of December 31, according to a study by Equilar, a corporate research firm.
That's up from 14 per cent a year earlier and from 12 per cent in 2013.
So, the trend is toward more equal representation on boards, but parity won't happen until the end of 2055 unless the pace picks up, according to Equilar.
That's nearly 40 years away, which may be about when girls born today begin sitting on corporate boards.
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Even so, it's still much easier to find a woman in the boardroom than in the corner office, according to a separate, global survey of 3,400 companies by Credit Suisse.
While women occupied nearly 15 per cent of board seats at the end of 2015, only about 4 per cent of CEOs are women.
Companies in other countries have gotten closer to gender parity than the United States, and government pressure has played a big role. Several European countries have set quotas and targets for how many corporate board members should be held by women.
Regardless, having women in the most senior leadership positions has other benefits, companies say.
At American Water Works Co, the largest publicly traded US water and wastewater utility, five of the nine board positions are held by women.
"We want to look like the customers and the employees that we serve in our local markets," she says. "The board, starting in 2012, really began to focus on assuring that they were diverse and looked like and represented the company."
American Water's majority-female board is an anomaly. Just 21 of the 3,525 companies in Equilar's survey have half or more of their board seats held by women. Equilar based its study on the Russell 3000 index, a broad measure of publicly traded US companies.
Like American Water, Connecticut Water is another utility that has a majority-female board. It also says it didn't start with the explicit intent of having more women than men on its board. Instead, it happened as the byproduct of looking for a more diverse group to represent its shareholders, customers and employees. Five of the company's eight director seats are held by women.
"It's a bit of an arduous process, looking for someone that fulfills what we're driving for in our cultural focus," says Kristen Johnson, corporate secretary and vice president of human resources at Connecticut Water. "We've been super-lucky to find individuals that populate the board like that, and it just so happens that five of them are women.