There's not much evidence into the impact of screen time – either positive or negative, says Dr Suzi Gage, a psychologist at the University of Liverpool. There are several studies, but the results they give are unclear and difficult to interpret. "It's not that the studies are bad," she says. "It's that they're hard to do."
Essentially, say you looked at 10,000 people who use screens lots and lots, and 10,000 people who hardly use screens at all, and you found (say) that the people who use screens lots and lots are happier than people who don't. Does that show you that screens make you happier?
Well, no. Maybe it just happens that people who use screens a lot are happier anyway. Maybe they're richer, on average, and maybe that makes them happier. You can try to "control" for that, by comparing people against others of similar income, but there still could be something you haven't thought of. "People who use screens might be different in ways that we don't always understand," says Gage. "It's hard to know what causes a correlation."
That's a common problem in this sort of research. But it's even harder with screens, because the direction of causality can get so mixed up. If people who are socially anxious use screens more, is that because the screens are making them anxious? "Perhaps they use screens more because they find it hard interacting with people face-to-face," says Gage. If that's the case, then it could be that screens actively improve such people's social lives rather than hindering them.
There's also the problem that it may not be the screens that are the problem, but the time they're taking up. "What's difficult to know, when people are using screens more, is: What aren't they doing instead?" asks Gage. "Are they sleeping less? Are they going outside less? Are they socialising less? There could be all sorts of things they're missing out on by using screens. Untangling that from the screen time itself is hard."