The notion that boys are better at math simply doesn't add up, according to a study to be published today in the journal Science.

An analysis of standardized test scores from more than 7.2 million U.S. students in grades 2 through 11 found no difference in math scores for girls and boys, contradicting a pervasive belief that most women aren't hard-wired for careers in science and technology.

Studies 20 years ago showed girls and boys did equally well on math in elementary school, but girls fell behind in high school.

The study's authors at the University of California-Berkeley and University of Wisconsin-Madison offered several theories for the improvements, including changes in educational approaches and career expectations.

"Stereotypes are very, very resistant to change, but as a scientist I have to challenge them with data," said Wisconsin's Janet S. Hyde, a psychology professor and the lead investigator of the study.

Hyde and her colleagues examined data from math tests administered from 2005 to 2007 as part of the No Child Left Behind initiative, which mandates annual testing of students from elementary school through high school. Comparing the average scores of girls and boys in 10 states, the researchers found that neither gender consistently outpaced the other in any state or at any grade level.

Among math whizzes, there remain sex differences.

But they don't add up to anything definitive. For instance, there are more white boys than girls with scores in the 99th percentile. But among Asian-Americans, it's reversed: Girls outperform boys. Reliable data was not available for Hispanics, blacks and American Indians.

In a separate analysis, they found that a gap persists in one important test, the Scholastic Aptitude Test.

However, the researchers pointed out that the SAT is not an indication of overall ability, because it is not administered to a random sample of students.

What's more, far more girls take the SAT than boys, "so you're dipping further down into the female talent pool, which brings down the average score," Hyde said. "That may be the explanation for (the results), rather than girls aren't as good as boys in math."

For the class of 2007, the latest figures available, boys scored an average of 533 on the math section of the SAT, compared with 499 for girls.

On the ACT, another test on which girls lag slightly, the gender gap disappeared in Colorado and Illinois once state officials required all students to take the test.

McClatchy-Tribune News Service, the Los Angeles Times and Associated Press contributed to this report.