WASHINGTON — Women, who make up half the American workforce, may be 40 years or more from parity with men on US corporate boards, the Government Accountability Office reported.

About 23 percent of open seats at companies in the Standard & Poor’s 1500 index went to women in 2014, according to the GAO. If that figure were to rise to about 50 percent, boards would be evenly split between women and men by roughly 2055, the report said.

Women held about 16 percent of board seats in the S&P 1500 in 2014, up from 8 percent in 1997.

“There are ways we can move faster,’’ said Susan Stautberg, chairman and chief executive of the Women Corporate Directors Foundation, which advocates for more women board members. “We can set goals, we can focus on mentorships and succession. You can’t just sit back and play it safe anymore.’’

European countries, including Sweden and France, have imposed quotas to get more women on boards, and the United Kingdom has moved the needle with a nonbinding push from the government and public commitments by corporate chairmen.

The United States has resisted regulation to speed up gender equality on boards. More than 350 of 1,846 public companies have no women, and another 628 have only one, according to 2020 Women on Boards, a Boston group.

The GAO, which did not make recommendations in its 38-page report, said several factors inhibit more rapid change, including a lack of women in executive positions and low turnover of board seats. About 600 board seats change hands yearly on the S&P 1500, or about 4 percent of the total.

The report highlighted the Securities and Exchange Commission’s effort to study whether better company disclosure of board and leadership diversity would help speed the process. Funds representing about $1.12 trillion in investments, including the California Public Employees Retirement System and the New York City comptroller’s office, petitioned the SEC in March to require more disclosure, including gender details, for board nominees.

Other suggestions included requiring candidate slates for boards to include at least one woman; setting voluntary targets; adopting term and age limits for directors; and expanding boards to add more women, the GAO said, based on interviews with 19 executives, governance experts, and investors conducted as part of the study.

In the smaller S&P 500 index, women made up 31 percent of new directors last year, taking 117 of the 376 board seats that changed hands, the most ever in a year, according to executive recruiter Spencer Stuart. Women held 22, or 4.4 percent, of the CEO jobs in that index.

Women are also moving to the top faster in family-run companies, Stautberg said. Those dominated by hedge funds, private equity, and venture capital remain far less diverse in the boardroom, she added.

“If we don’t get this fixed, we’re going to be left behind by companies in other countries that are doing it,’’ Stautberg said.