Researchers have found one more clue in their search for the reason that girls don't do as well as boys in math: a nation's culture.
Scientists compared math and reading scores on tests given to thousands of 15-year-old students in 40 countries and then examined how each country ranked in terms of gender equality.
While girls generally scored lower in math than boys, girls did better in countries with greater gender equality than in less progressive countries.
Girls performed best in countries such as Norway and Iceland, which have progressive gender policies, and worst in countries such as Turkey, which scored relatively low on standard measures of gender equality. The U.S. fell somewhere in the middle, the researchers say.
Girls scored higher in reading everywhere - and their reading advantage widened in countries with more gender equality, according to the report in today's edition of the journal Science. But only in Iceland, a country known for its homogeneous population, did girls score better than boys in both reading and math.
Experts said that without more studies, they could not explain why girls' performance there was so far ahead of their counterparts in other countries. "All I can say is, don't mess with the girls from Iceland," said Luigi Zingales, a finance professor at the University of Chicago who is a co-author of the report.
There has been a variety of theories to explain the gender gap over the years - some biological and some psychological.
"We really don't know," Zingales said.
The researchers used 2003 results from the Program for International Student Assessment, which tested 276,000 children representing a cross section of each participating country's overall population. Math skills tested included basic geometry, algebra, arithmetic and probabilities.
Gender equality profiles were determined by measures such as the World Economic Forum's gender gap index, which ranks countries based on economic and political opportunities for women, and on other barometers such as longevity rates. The U.S. ranked 23rd of 128 countries in the WEF's 2006 analysis.
Girls in the U.S. scored an average 10 points lower than boys in math, which was about the international average.
Maryland educators say the study highlights the importance of cultural attitudes in the choices students make when deciding on high school courses and career tracks.
"The gender gap in math has been closing over time," said Francis "Skip" Fennel, an education professor at McDaniel College and a former president of the National Council of Teachers of Math. "There's been an acceptance of everybody learning math."
Still, girls are sometimes discouraged from excelling or pushing themselves in math, experts say.
"In the culture of the high school, the girls don't want to look too nerdy," said Henry Kepner, a former math teacher who is president of the National Council of Teachers of Math.
But Kepner and other experts say attitudes are changing and more girls are taking challenging math courses and testing their skills on national Advanced Placement exams in calculus and in statistics.
"Over the last 10 years or so, there has been a definite improvement in more women taking math," said Victoria Stevenson, head of the math department at Baltimore's all-girl academic magnet, Western High School.
At Western, increasing numbers of girls are signing up to take calculus in their senior year, Stevenson said. This year, 18 seniors are taking either of two calculus courses at Western - one of which wasn't even offered a few years ago, she said. She expects about 25 students to sign up for calculus next year.
"I think it's society today. There's a look toward technology as being a key and more jobs are available to women in engineering and technology," she said.
Among 2007 high school graduates nationwide, some 125,000 girls and 142,000 boys took the College Board's Advanced Placement calculus exam last year, regarded as the gold standard for students serious about careers in engineering, science and math. That's about twice the number of test takers as there were 10 years ago - and roughly the same proportions by gender. "That's important because these are the people who are stepping forward and saying, 'I'm ready for this exam,' " Kepner said.
But the average girl's math SAT score is still 35 points lower than he boy's score. That's because proportionally more girls are taking the SAT - a lower-level test than the calculus exam - creating a larger pool of test takers that pulls the average down, according to Wayne Camara, vice president of research for the College Board, which administers the test.
Among last year's seniors, 641,000 girls took the SATs, compared to only 533,000 boys. "That's a significant difference," Camara said.
Historically, studies show girls are better at writing, reading and the language arts, while boys perform better in the natural sciences, social sciences and math, Camara said. The differences begin to appear in the fourth grade, but increase gradually as students progress through the 12th grade, he said.
"As students go on in school, the differences get exacerbated. The gaps are bigger the more advanced the student," he said.
With fewer boys than girls taking the SATs and going to college, much of the concern about a gender gap in education today days focuses on how badly boys are faring.
About 57 percent of today's U.S. college students are female, experts say. Among the 40 percent of the nation's undergraduates who are 25 or younger, women outnumber men by almost 2-1, according to a 2006 report by the American Council on Education.
Boys also score uniformly lower in reading tests, in the United States and elsewhere, a result confirmed in the Science study.
"It's the boys in reading, I'm worried about. I don't know why they aren't moving up" said David Sadker, an education professor at American University specializing in gender issues in education.
Zingales, the co-author of today's study, jokingly agreed. "We are the weaker sex," he said
Although gender may matter, household income plays a much greater role in most standardized test score results, some experts say.
"What you see consistently is - for both boys and girls - those with higher incomes consistently score higher than lower-income boys or girls," said Christianne Corbett, a research associate with the American Association of University Women.
Meanwhile, with the exception of computer sciences, increasing numbers of women are entering science, technology, engineering and math-related fields, providing more role models for girls interested in math, Corbett noted.
"Kids tend to see as options things that are being done by people like themselves, so more women in these fields is important," she said.