"Contrary to the stereotypes that are held by many parents and teachers, we simply found no gender differences at any grade level in any state," says Janet Hyde, a psychology professor at the University of Wisconsin and one of the authors of the study, which appears in the journal Science.

Previous research had shown that while girls and boys had similar math performance in elementary school, boys pulled slightly ahead on more complex math problems about the time they reached high school, Hyde says.

But those findings came from test scores gathered two decades ago. Hyde was inspired to revisit this issue after some controversial remarks made in 2005 by Harvard University's then-president, Lawrence Summers, who had been discussing the possible reasons why women are underrepresented in fields such as physics and engineering.

Hyde and her colleagues wrote to the department of education in every state, requesting their most recent math scores from assessment tests. "We had a bonanza of data because of No Child Left Behind legislation, which mandates that all states have to test all students annually on their math performance," says Hyde.

Ten states across the country responded: California, Connecticut, Indiana, Kentucky, Minnesota, Missouri, New Jersey, New Mexico, West Virginia and Wyoming. They sent in scores for children in grades 2 through 11.

Hyde says the result was clear in all states and all grades: "Girls have reached parity with boys in math performance."

But her research team also found that the tests used for these state assessments didn't have a lot of really complex math problems. So Hyde looked at recent data from another testing program: the National Assessment of Educational Progress. It had tougher questions, she says, but "once again we had gender similarities."

Other researchers say the study is convincing. "The data that they're presenting sort of documents the fact that, for most math problems that are likely to be confronted in typical courses that kids take in the high school years, we have essentially wiped out the gender differences that were there before," says Jacquelynne Eccles, a psychology professor at the University of Michigan, Ann Arbor.

Eccles believes girls have closed the gap because they are increasingly choosing to take the toughest math classes available.

One math teacher, Fred Dillon from Strongsville High School in Ohio, says that's certainly been his experience over his 31 years of teaching. "I have seen in my classes that the number of females in the elective courses I teach, like an AP calculus or something, has increased steadily over the years," Dillon says. "It's pretty much 50-50 all the time now."

One student who's taken those kinds of classes is Patricia Li, of San Jose, Calif. She's going to be a high school senior this fall, but she's spending the summer at MIT doing math research. Li says she's never really gotten a vibe that it's weird for a girl to excel at math.

"I don't think my parents have any of those stereotypes," Li says, "and from teachers, I don't think so. From other students … maybe."

But she's not so sure that reaction from her peers is related to gender. "In school, I think the issue is more like that people in general don't like math," Li says, "rather than [that] boys are better at math than girls."

Melanie Matchett Wood, a graduate student in mathematics at Princeton University who was the first female to make the prestigious U. S. International Math Olympiad Team, agrees that math is generally seen as being nerdy.

But Wood thinks that "nerdiness in our society isn't gender neutral. It's much easier for boys to find a way to be a nerd than it is for girls."

Last year, Wood coached Li when she was on a team of high school students sponsored by the Mathematical Sciences Research Institute that went to high-level math competition for female students, the 2007 China Girls Mathematical Olympiad. It was the first time the U.S. had sent a team to this event.

"Getting to see these eight young women who all love math working together as a team, that was amazing," Wood says. "When I was a student, I never had an experience like that. Even now, I'm rarely in a room with two or three other women math colleagues. Seven would be incredible."

Wood thinks if research is showing that girls and boys now perform equally well on math in school, researchers need to better understand why females still seem less likely to pursue a career in math-intensive technology and science fields.