WASHINGTON — Boys outperform girls on a math test given to children worldwide, but the gender gap is less pronounced in countries where women and men have similar rights and opportunities, according to a study published yesterday.

"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, 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.

The United States fell in the middle of the pack in terms of both equality for women and the gender gap in math.

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.