It has almost become an accepted truth, with social media bans becoming increasingly popular among politicians. Young people spend more and more time on social media. Mental health appears to be declining. So one must be causing the other.

Over the past few years, numerous studies have reported an association between heavier social media use and higher levels of depression or anxiety, other didn’t. But what happens if, instead of following young people for a few months, you follow them for five years?

A new Swedish cohort study did exactly that. Martina Zetterqvist and colleagues followed more than 3,000 Swedish adolescents. At age fifteen, they asked how much time they spent on social media. Five years later, they measured their mental health. That makes this study more informative than much of the earlier research, which often relied on cross-sectional data or followed participants for only a relatively short period.

At first glance, the story seems straightforward. Adolescents who spent more time on social media at age fifteen reported slightly higher levels of anxiety and depressive symptoms five years later.

But that finding did not hold up. Once the researchers accounted for differences that already existed when the participants were fifteen, the association disappeared. Some adolescents already had more emotional problems, were more impulsive, came from different socioeconomic backgrounds, or received more or less parental support. Those factors turned out to matter much more. Time spent on social media, by itself, no longer predicted mental health five years later.

Perhaps this only applied to certain groups? Girls, for example? Or adolescents from more disadvantaged backgrounds?

Again, the answer was no. The association was no stronger for girls, for adolescents with a lower socioeconomic background, or for those who were less physically active.

Does this mean social media have no effect on mental health?

No. That is not what this study shows.

Its conclusion is much narrower. The amount of time adolescents spend on social media is, by itself, not a good predictor of their mental health five years later.

That is perhaps less surprising than it sounds. We have seen something similar in research on video games. Time is often a very crude measure. Spending three hours chatting with friends is fundamentally different from spending three hours passively scrolling, comparing yourself to others, or being exposed to cyberbullying. Yet all of those activities count equally in a measure based solely on time.

The authors acknowledge this limitation themselves. Perhaps we have been measuring the wrong thing for years. Rather than asking how long adolescents spend on social media, the more interesting question may be what they actually do there. And, to be fair, many researchers have been making exactly that point for quite some time.