A new global study found that while kids who read anything excel academically, kids who read traditional paper books do best.

Reading lots of books is well-established as a pretty sure-fire way for kids to better excel in school and life. But does how they read those books make a difference? A new global study that tracked the reading habits of 600,000 kids found that reading is an amazing tool for all kids, but that reading good old paper books has significantly better outcomes than reading digital books.

The Organization for Economic Cooperation and Development (OECD) study looked at the results of the Program for International Student Assessment (PISA), which kids in 30 countries take every three years. The assessment includes a survey where kids share general information about themselves, their families, and their reading habits, making it an ideal opportunity for researchers to learn more about how kids around the world learn.

The latest assessment shows that kids who often read hard copies of books scored a whopping 49 points higher on the test than kids who read rarely — putting them 2.5 grade levels above their peers. Not only that, but kids who read paper books scored were almost a year ahead of those who read often but mostly read digital books.

Kids who read paper books also reported enjoying reading more than those who did not.

This was true even when corrected for demographic differences like economic background.

Researchers stress that this study correlates reading paper books with higher test scores — they still aren’t totally sure why paper books are better (although the same result has been found across age groups in at least 29 different studies). While the science shows that people generally comprehend and retain more from reading off of real paper, other factors could be at work. For example, maybe kids who love reading and school simply read more books in general, which includes a mix of paper and digital material.

It’s important to note, though, that even in countries where screens are much more popular than paper books when it comes to long-form reading — like Hong Kong, Malaysia, and Taiwan — kids who read traditional books still did better on assessment tests.

Why is knowing that paper trumps screens when it comes to reading important to kids’ education, overall?

Researchers tied to the study are worried that kids living in poverty and kids with less privilege currently have better access to books via screens, which are less expensive and easier to get, while they have increasingly less access to traditional books. And that if they don’t have access to hard copy books, their reliance on digital books could actually open a new gap between kids who have resources and kids who don’t.

“Even though much of the world has become increasingly digitalized,” the researchers wrote in their report, “the issue of equal access to print books should not be forgotten. While disadvantaged students are catching up in terms of access to digital resources, their access to cultural capital like paper books at home has diminished and the socio-economic gap has been persistent.”

This is solidified by an additional finding from the study that kids who read paper books generally have more paper books surrounding them at home.

In other words, as educators learn more about what kinds of formats benefit kids most, we need to make sure that everyone has access to that specific format — so everyone can have a chance to discover their love of reading and excel at school.