While representation of women on the boards of U.S. publicly-traded companies has been increasing, equality could still be decades away, according to a new report by the Government Accountability Office.

In 2014, women accounted for about 16% of board seats in the S&P 1500, a ratio that has increased steadily from 8% in 1997. However, even if equal proportions of women and men joined boards each year beginning in 2015, the GAO estimated it could take more than four decades for women’s representation on boards to be on par with that of men’s.

“Even if every future board vacancy were filled by a woman, we estimated that it would take until 2024 for women to approach parity with men in the boardroom,” the report said.

The main obstacles to gender parity include boards not prioritizing recruiting diverse candidates; few women in the traditional pipeline to board service, particularly with chief executive or board experience; and low turnover of board seats.

“New executives are chosen by the board members’ personal networks. And if birds of a feather flock together, women will most likely not be included on the list,” Jan Babiak, an executive coach and independent director on several corporate boards, told CNBC.

The GAO interviewed CEOs, board directors, and public pension fund investors, the majority of whom said they generally preferred voluntary strategies for increasing gender diversity on corporate boards. However, several large investors and most stakeholders supported improving U.S. Securities and Exchange Commission disclosure requirements on board diversity.

“Institutional investors driving this push for disclosure is a step in the right direction,” The Chicago Network president and CEO Kate Bensen told CNBC. “I think that making sure women who are ‘board ready’ become ‘board eligible’ is also a way to alleviate the gender gap problem. It is necessary that search firms have women in their pipeline who are ready to fulfill those roles.”