What’s going on, exactly? The short answer is that no one knows for sure. One possibility is that the 2008 financial crisis, a genuinely world-historical event, had some medium- and long-term effects. Think of the millions of kids raised in households hit with a sudden jolt of genuine, novel precarity — whether from a home foreclosure forcing a move, a parent losing a job or other factors — and then having to live with the aftermath. Researchers have believed for a long time that traumatic events suffered at a young age have a particularly brutal impact on young people’s wellbeing. It makes sound theoretical sense that the recession would leave an impact on the children hit hardest by it, and that the effects would linger for years. (Of course it left an impact on adults as well — by one 2013 estimate published in the British Medical Journal, the largest suicide rate increase in the Americas occurred among men aged 45-64, a group hard hit by layoffs and foreclosures.)

And not long after the recession, another world-historical development: the explosive rise of social media on mobile devices. Facebook accessed via a shared clunker of a desktop computer in the family room on dial-up is one thing; Facebook on a smartphone is quite another. Starting around 2010, teenagers were able to mainline information about their classmates, and to communicate with them 24/7, in a manner never before possible.

Researchers like Haidt believe that this can go a long way toward explaining the subsequent uptick in youth mental-health concerns. “A mean tweet doesn’t kill anyone; it is an attempt to shame or punish someone publicly while broadcasting one’s own virtue, brilliance, or tribal loyalties,” he wrote recently. “It’s more a dart than a bullet, causing pain but no fatalities. Even so, from 2009 to 2012, Facebook and Twitter passed out roughly 1 billion dart guns globally.” And Haidt thinks teenage girls are more particularly vulnerable to this dart warfare — more often on Instagram than Twitter, in their case — and that the effects on their mental health have been, in retrospect, predictable.