Controversial new regulations give educators far more latitude to establish schools and classes strictly for a single gender, even as research on the practice is scarce and inconclusive.
The regulations, released Wednesday by the Department of Education, mark a major shift in the interpretation of Title IX, approved 34 years ago to bar sex discrimination in schools.
It's a change that has intensified a long-running debate over whether boys and girls learn better in a single-sex environment, with critics warning the regulations may roll back years of hard-won ground.
Even the Department of Education, in announcing the rules, acknowledged research is mixed and backed away from endorsing single-sex classrooms.
"The research, though it's ongoing and shows mixed results, suggests that single-sex education can provide benefits to some students under certain circumstances," said Assistant Secretary of Education Stephanie Monroe, in a news briefing. She emphasized any single-sex environment would be voluntary, and an equivalent coeducational option would be available.
Research on the practice has been controversial. Theories that each gender has different learning styles or brain growth, or that boys are losing ground in traditional schools, have caught on in the media and popular imagination.
However critics say little of it stands up to scrutiny, and there are far more similarities between genders – and differences among individuals – than there are broad general differences between the sexes.
"Race and class are the two biggest predictors [of achievement] in every single study I've looked at," says Rosalind Barnett, a senior scientist at the Women's Studies Research Center at Brandeis University, in Waltham, Mass. "Of all the things you could think about doing to improve educational outcomes, separating kids by gender is really low on the list."
Dr. Barnett questions using resources for something with so little scientific basis, and she worries there could be negative consequences if girls and boys start to believe what she says are myths of gender differences – that girls are challenged in math and science, and boys have a harder time with reading and verbal skills.
Nonetheless, single-sex classrooms are catching on among many parents and educators who feel they see a difference in kids and believe it might help them focus.
Public school districts have held off on doing much that's separated by gender for fear of legal challenges, but a few single-sex schools have opened in recent years as pilot programs or if the district could show a compelling reason for doing so.
Some 241 public schools now offer some single-sex classrooms, up from three in 1995, according to the National Association for Single Sex Public Education (NASSPE). Of those 51 are completely single-gender schools.
The Minneapolis Academy, a small charter school in Minnesota that opened two years ago, offers only single-sex classes for its seventh- and eighth-graders. Academy director Leon Cooper says he set out to create a school with high expectations for inner-city kids who typically start out behind grade level. He added the single-sex classes "on gut instinct" after seeing how the practice worked at private schools he visited.
The school has had success at bringing kids back up to grade level, he says.
"This separation is not segregation," Mr. Cooper says. "These kids have all kinds of social interaction, but during the day the reason to be here is not to interact socially, it's to learn to read and write."
Some advocates, aware of the scanty research, are pushing for the option for reasons of social justice and parental choice, more than because of differences in learning styles or brain development.
"Parents with lots of money can choose single-sex schools, so why can't parents who don't have a lot of money have the same kind of choice?" asks Leonard Sax, director of the NASSPE.
Critics dismiss that quickly.
"You could say that parents could choose to send their kids to racially segregated schools as well, but that isn't something we'd want to have in the public school system," says Kim Gandy, president of the National Organization for Women.
Other advocates say even if the focus on gender differences in learning is not reason enough for change, there may be social reasons.
"For disadvantaged students, they don't necessarily identify with academic achievement, and for many families, the choice of a single-sex school is a very pro-academic choice," says Rosemary Salomone, a law professor at St. John's University in New York and author of "Same, Different, Equal: Rethinking Single-Sex Schooling."
"You're saying, particularly to teenagers, school is a very serious business," she says. "It frees them from the social distractions of the other sex."