'In more gender-neutral societies, girls are as good as boys in mathematics,' study author Paola Sapienza said in an interview.
The issue of a gender gap in math has been hotly debated, with some suggesting biology may be behind higher scores for boys on some tests and others pointing to environmental and cultural factors.
Sapienza, a professor at Northwestern University's Kellogg School of Management, examined the results of boys and girls on the Program for International Student Assessment. That test is given to 15-year-olds around the world every three years.
Among 40 countries studied, Iceland was the only one where girls did better than boys on the math test.
In about a dozen countries, both sexes scored about the same. In many of those places, like in Iceland, men and women have similar opportunities and rights, according to the study, which was published in the journal Science.
To assess gender equality, Sapienza looked at several measures, including the World Economic Forum's Gender Gap Index, which considers economic, educational and political opportunities for women.
The United States fell in the middle of the pack in terms of both equality for women and the gender gap in math.
There are a few countries where girls don't have the same opportunities as boys, but girls score about the same as boys on the math test, the report found. These included Indonesia and Thailand.
There also are countries, such as Germany, where there is a lot of gender equality but where a girl-boy math gap exists anyway. The study did not attempt to explain such anomalies.
The study was reviewed by a panel of outside experts.
In reading, girls outperformed boys on the PISA exam in every country studied. That gap does not shrink but widens in places where women are said to have a lot of equality with men.
“The math gap disappears, and the reading gap becomes even bigger,” Sapienza said.
The study didn't look at the reasons behind those trends.
Former Harvard University President Lawrence Summers ignited debate over gender gaps a few years ago year when he suggested innate differences between the sexes might help explain why comparatively few women excelled in science and math careers.
The PISA exam is different from other tests in that it assesses how well students can apply mathematical reasoning to real-world situations rather than testing their knowledge of math content.