Girls do better than boys in all school subjects, according to a recent study published by the American Psychological Association.

Despite the stereotype that boys do better in math and science, researchers found that girls have made higher grades than boys throughout their school years for nearly a century.

"Although gender differences follow essentially stereotypical patterns on achievement tests in which boys typically score higher on math and science, females have the advantage on school grades regardless of the material," Daniel Voyer, lead author of the study, said in a statement. "School marks reflect learning in the larger social context of the classroom and require effort and persistence over long periods of time, whereas standardized tests assess basic or specialized academic abilities and aptitudes at one point in time without social influences."

For the study, researchers examined 369 samples from 308 studies, reflecting grades of 538,710 boys and 595,332 girls. The research spanned more than 30 countries.  Seventy percent of the samples consisted of students from the United States. Other countries or regions represented by more than one sample included Norway, Canada, and Hong Kong.

Researchers found that the differences in grades between girls and boys were largest for language courses and smallest for math and science. The female advantage in school performance in math and science did not become apparent until junior or middle school, according to the study.

 The degree of gender difference in grades increased from elementary to middle school, but decreased between high school and college.

The study also reveals that recent claims of a "boy crisis," with boys lagging behind girls in school achievement, are not accurate because girls' grades have been consistently higher than boys' across several decades with no significant changes in recent years.

"The fact that females generally perform better than their male counterparts throughout what is essentially mandatory schooling in most countries seems to be a well-kept secret, considering how little attention it has received as a global phenomenon," Susan Voyer, co-author of the study, said in a statement.

Researchers believe social and cultural factors could be among several possible explanations for why girls perform better in school than boys, the authors speculated that.

Parents may assume boys are better at math and science so they might encourage girls to put more effort into their studies, which could lead to the slight advantage girls have in all courses, researchers said. Gender differences in learning styles are another possibility.

Previous research has shown girls tend to study in order to understand the materials, whereas boys emphasize performance, which indicates a focus on the final grades.

"Mastery of the subject matter generally produces better marks than performance emphasis, so this could account in part for males' lower marks than females," the authors wrote.

The findings were recently published in the APA journal Psychological Bulletin.