Over the typical lifetime, happiness tends to follow a U-shaped curve, peaking at 30, plummeting at age 50, before spiking again after 70. It's a pattern replicated using data going back as far as the 1970s in almost 150 countries. But around 2011, researchers noticed an astonishing reversal in this trend. "This empirical regularity has been replaced by a monotonic decrease in illbeing by age," they reported in an NBER working paper. In plain English, younger people today are unhappier, both compared to previous generations and to their older peers. In the US, for example, reported rates of anxiety among young people have exploded. So too have emergency room visits for self-harm. Similar trends can be seen in places like other English-speaking countries and the Nordics. "Why did it happen all over the world?" [New York University's Jonathan] Haidt asked before sharing the theory he also puts forward in his best-selling book: "We have over-protected children in the real world and under-protected them online." Today, rather than playing with their friends, kids stay at home on their devices. Instead of hearing chatter and laughter in the corridor of schools, we hear the gentle tapping of screens. In the UK, as part of the Smartphone Free Childhood grassroots movement, 85,000 parents have signed a pact committing to delay giving their child a smartphone. To date, parents in 25 countries, from Argentina to Uzbekistan, have joined the movement.