In school, boys are falling behind. Around the world, they're more likely than girls to be classified as low achievers, meaning they weren't proficient in reading, math, science, or problem-solving.

The OECD looked at the data from the test and an associated survey to try to figure out what's causing boys to lose ground. Here are five of their findings:

1) Boys are more likely to be disengaged at school

Across the developed world, 15-year-old boys were more likely than girls to say that school is a waste of time and less likely to agree that trying hard at school is important. Boys are more likely to be late to class or skip school entirely. And boys are more likely to be held back than girls are.

These differences aren't massive — in most cases, they're less than 10 percentage points. But they still indicate that there are big, gender-based differences in how students think about school and how important they think it is.

2) There's a gender gap on doing homework

It doesn't matter whether students have a lot of homework (almost 15 hours per week in Shanghai) or very little (around 3 hours per week in Finland): boys spent less time on it than girls. On average in the OECD, girls reported doing 5.5 hours of homework per week; boys did only 4.5.

That doesn't explain all of the gender gap in test scores, but it does explain part of it, according to the OECD — after accounting for the time spent doing homework, boys actually perform better than girls in math and science, and the gender gap in reading is smaller.

3) Boys spend more time on the internet and playing video games

What are boys doing with all that non-homework time? They're on the internet and playing video games, the survey suggests. Most girls say they never or have hardly ever played single-player or collaborative online games. On the other hand, most boys do, but not every day.

Is this a disadvantage? It might not be. Playing single-player games was correlated with higher test scores, although collaborative games was associated with lower scores. And boys tend to do better than girls on standardized tests taken on the computer, rather than on paper. That suggests the video gaming time could help them as computer-based assessments become more common.

4) Boys read for fun much less than girls

The gender gap between boys and girls is particularly evident in one subject: reading. And in every country but Korea, girls do more reading for fun than boys. The OECD found that it didn't really matter what students read — whether it was fiction, nonfiction, newspapers, or comic books, more time spent reading translated into higher reading test scores. Boys were more likely to read comic books and newspapers, and less likely to read fiction. But elementary school reading in particular has historically focused on nonfiction, and the OECD argues that this could be turning boys off of reading and widening a gender gap.

5) Boys have lower aspirations for their careers

Across the world, 15-year-old girls expect to have higher-status jobs when they're older than boys do. They're more likely to expect to earn a college degree, and more likely to say they expect to have high-status jobs as managers, professionals, or elected officials. On the other hand, the percentage of men and women in those jobs in their mid-20s to early 30s is almost exactly the same. That suggests that girls are more ambitious — but they're not necessarily achieving those goals.