By Linda Feldmann

Washington - The only two women ever to have served on the US Supreme Court - Justices Sandra Day O'Connor and Ruth Bader Ginsburg - have a saying: "At the end of the day, a wise old man and a wise old woman reach the same judgment."

But there's a "but." Justice Ginsburg herself speaks of the perceptions women have because they are women. And a recent study finds that, at least in sex-discrimination cases, there is a difference between how male and female judges rule.

The nine-member high court is down to one woman after Justice O'Connor retired and was replaced in 2005 by Justice Samuel Alito. Speculation is strong that President Obama will nominate a woman to replace retiring Justice David Souter. Women dominate the lists of names Mr. Obama is reportedly considering.

For advocates of women's rights, adding another female justice is not about diversity for diversity's sake. It's about bringing women's perspectives and life experiences into interpretations of law, and about helping the male justices see things through their eyes.

Eleanor Smeal, president of the Feminist Majority Foundation, says that when she first began pushing to get more women into a variety of professions, including law and politics, she assumed that they would behave similarly to people of their ideological bent. Indeed, a conservative woman rules and votes differently from a liberal woman.

"But all things being the same, I discovered that a conservative woman acts more pro-women's rights than a conservative man," Ms. Smeal says.

A recent study by three academics focusing on the votes of federal court of appeals judges bears out this observation. The research, conducted by Lee Epstein of Northwestern University Law School in Chicago and Christina Boyd and Andrew Martin of Washington University in St. Louis, found that on most issues, there was no difference in the voting patterns of male and female judges. But in sex-discrimination cases, female judges were about 10 percent more likely to rule for the party bringing the suit.

Female appeals-court judges also appeared to have an impact on their male colleagues. The study found that when male and female judges sit together on a sex-discrimination case, the men are almost 15 percent more likely to rule for the plaintiff than when only men are ruling. The study controlled for ideological leanings.

"If Obama is considering two fairly moderate people, one a woman and the other a man, we would expect the woman to cast more liberal votes in sex-discrimination cases," two of the researchers wrote in The Washington Post on May 3. "The same would be true if the president were considering two very liberal candidates, again, one a man and one a woman.

The recently argued Supreme Court case over the strip-search of a 13-year-old girl, though not a sex-discrimination case, illustrates how often-like-minded judges of opposite sexes can see things differently.

During the argument, Ginsburg expressed indignation at the idea of an adolescent girl being asked to shake out her bra and panties in front of school administrators.

Justice Stephen Breyer seemed to shrug. "In my experience, when I was eight or 10 or 12 years old, you know, we did take our clothes off once a day, we changed for gym, OK?" he said.

In an interview later with USA Today, Ginsburg elaborated on her perspective in the case - and that of some of the male justices. "They have never been a 13-year-old girl," the justice said. "It's a very sensitive age for a girl. I didn't think that my colleagues, some of them, quite understood."

Ginsburg and O'Connor have spoken at length about the discrimination they experienced as law students and as young lawyers trying to launch a career. Today, nearly half of law students are female, as are nearly one-third of American lawyers. Women's groups hoping that Obama replaces Justice Souter with a woman say he has plenty of excellent choices.

If Obama doesn't choose a woman, will he face repercussions from his liberal base and the legions of women who supported him?

"There would be an awful lot of disappointed people around the country, men and women," says Marcia Greenberger, co-president of the National Women's Law Center. "Like in any other situation, there'd be scrutiny on the person, and that's going to be true on a woman or a man."

Though Obama aides have made clear that overt lobbying could be counterproductive, Ms. Greenberger said the White House has been open to ideas and suggestions.

Not all women think the sex of the next justice should be of paramount concern. In this day and age, with a black president, the third woman as secretary of State, and a diverse Cabinet, Obama should look foremost at qualifications, says Carrie Lukas, vice president for policy and economics at the Independent Women's Forum.

"You don't want everyone to have the exact same background and the exact same set of experiences, because we do know that that does influence how people think," says Ms. Lukas. "But I just hope that we're in the process of moving beyond identity politics every time there's a major appointment."