Something is going very wrong for teenagers. Between 1994 and 2010, the share of British teens who do not consider themselves likeable fell slightly from 6 per cent to 4 per cent; since 2010 it has more than doubled. The share who think of themselves as a failure, who worry a lot and who are dissatisfied with their lives also kicked up sharply.

The same trends are visible across the Atlantic. The number of US high school students who say their life often feels meaningless has rocketed in the past 12 years. And it’s not just the anglosphere. In France, rates of depression among 15- to 24-year-olds have quadrupled in the past decade.

Wherever you look, youth mental health is collapsing, and the inflection point is ominously consistent: 2010 give or take a year or two — when smartphones went from luxury to ubiquity.

The theory that having social media and other digital delights within arm’s reach 24/7 may be having a harmful effect on mental health is not new. Its leading advocate is Jean Twenge, professor of psychology at San Diego State University and author of dozens of pioneering studies on the topic.

But it is still far from universally accepted. The work of Twenge and her regular co-author Jonathan Haidt has at times been criticised for simply surfing the wave of popular opposition to big tech. Yet as evidence for their arguments mounts, many are now wondering why it has taken us so long to accept what was right in front of us.

The signs are everywhere. First, digital socialising has displaced in-person gatherings. The share of US teens who meet up in-person with friends less than once a month stood at 3 per cent between 1990 and 2010, but reached 10 per cent by 2019, meanwhile the share who say they are “constantly online” has now reached 46 per cent.

Some counter that it can’t just be that apps are crowding out real life — after all, the people who are busiest on Instagram are often the busiest in the real world, too. But that misses a key dynamic: these trends operate at the generational level, not the individual. As screen-time has surged, everyone hangs out less.

But the individual-level dynamics are striking, too. Studies show that the more time teens spend on social media, the worse their mental health is. The gradient is steepest for girls, who also spend much more time on social media than boys, explaining the sharper deterioration among girls’ mental health than boys’.

It’s a similar story with the higher rates of depression among liberal teens than conservatives. If you suspect liberal kids are more depressed due to growing up in a culture that valorises concern for injustice, I would advise caution. First, Twenge’s research points to a likelier explanation: liberal youths simply spend more time online than conservatives. Second, we see the same rising trend among conservatives — it’s just lagging.

Some suggest that modern society is more open about discussing mental health, so what we’re seeing is just a rise in reporting, not prevalence. But British teens who spend five or more hours a day on social media are at two to three times greater risk of self-harm than their less-online peers. It’s a similar story in the US with suicidal ideation. Grimmest of all, the now-familiar hockey stick trend is also clear in rates of suicide deaths among British and American teens.

Others point out that correlation is not causation. Indeed. But we now have a growing body of research showing that reducing time on social media improves mental health.

So, what can we do? The most common response is “educate kids and parents”. But as the cases of obesity and smoking show, public information campaigns are notoriously ineffective in the face of addiction.

Another option would be to build on the evidence that when people are encouraged to take an extended break from social media, some disconnect for good. And then there’s regulation — why not increase the age limit for social apps and punish companies that don’t enforce them?

Ultimately, though, I’m not optimistic. Combating obesity has been so hard because you can’t stop people eating food. And fighting social media addiction is hard because you can’t stop people using smartphones and apps. Until someone invents the equivalent of a weight-loss drug for Instagram, the future looks ominous.

john.burn-murdoch@ft.com, @jburnmurdoch