WASHINGTON — At the rate they’re going, it will take another 107 years until women hold half the seats in Congress, according to a study by the Institute for Women’s Policy Research, a nonpartisan group that studies female-centered issues.

With 99 women among the 535 members of Congress currently – a record high – the 107-year estimate is “an optimistic model,” institute study director Jeffrey Hayes said.

In the last 20 years, the number of women in Congress has grown at a rate of one to nine female members per session. The incremental steps toward gender parity follow from the 1992 election, when the number of women in Congress rose from 32 to 54. That increase was the last of that magnitude.

The 1992 surge of victorious female candidates stemmed, in part, from a backlash after many of the members of the all-male Senate Judiciary Committee in 1991 brushed off Anita Hill’s claims that Supreme Court nominee Clarence Thomas had sexually harassed her.

As the numbers grew steadily if gradually, Congress changed.

Most notably, Rep. Nancy Pelosi, D-Calif., was elected the first female speaker of the House of Representatives in 2007. Another change: The House installed a more centrally located women’s restroom, near the Speaker’s Lobby, in 2011.

While a woman has never served as the minority or majority leader on the Senate side, Sen. Barbara Mikulski, D-Md., the first female chair of the Senate Appropriations Committee, became the longest-serving female senator in 2012.

Beyond the reasons both genders run for elected office, such as ambition and public service, women tend to run for additional reasons, according to Kathy Kleeman, senior communications officer at the Center for American Women and Politics at Rutgers University:

• An issue they’re interested in is dominated by men.

• They’ve been inspired or enabled by other female politicians.

“What has to happen is more women need to decide to run, and more needs to happen to get them to make that decision,” Kleeman said.