WASHINGTON - Sixteen years after Barbie dolls declared, "Math class is tough!" girls are proving that when it comes to math they are just as tough as boys.

In the largest study of its kind, girls measured up to boys in grades second through 11th. The research was released Thursday in the journal Science.

Parents and teachers persist in thinking boys are simply better at math, said Janet Hyde, the University of Wisconsin-Madison researcher who led the study. And girls who grow up believing it wind up avoiding harder math classes.

"It keeps girls and women out of a lot of careers, particularly high-prestige, lucrative careers in science and technology," Hyde said.

That's changing, though slowly.

Women are now earning 48 percent of undergraduate college degrees in math; they still lag far behind in physics and engineering.

But in primary and secondary school, girls have caught up, with researchers attributing that advance to increasing numbers of girls taking advanced math classes such as calculus.

Hyde and her colleagues looked at annual math tests required by the No Child Left Behind education law in 2002. Ten states provided enough statistical information to review test scores by sex, allowing researchers to compare the performances of more than 7 million children.

The researchers found no difference in the scores of boys versus girls -- not even in high school. Studies 20 years ago showed girls and boys did equally well on math in elementary school, but girls fell behind in high school.

"Girls have now achieved gender parity in performance on standardized math tests," Hyde said.

The stereotype that boys are better at math has been fueled, at least in part, by suggestions of biological differences in the way little boys and little girls learn. This idea is hotly disputed; Lawrence Summers, then the president of Harvard, was castigated in 2005 when he questioned the "intrinsic aptitude" of women for top-level math and science.

Joy Lee, a rising senior at Thomas Jefferson High School for Science and Technology in Alexandria, Va., says she always felt confident about math but remembers how it felt to walk into a science class full of boys. "Maybe I was a little bit apprehensive about being the only girl, but that didn't last for very long," said Lee, president of a school club that tries to get young girls interested in science and technology.

"I definitely do encourage other girls to pursue those interests and to not be scared to take those courses just because there are not very many girls or because they think they're not good enough to do it," Lee said.