After more than a decade of stability or improvement, the mental health of adolescents in many countries around the world deteriorated suddenly in the early 2010s. Why have rates of depression, anxiety, self-harm and suicide risen so sharply, more than doubling in many cases? In this book, Social Psychologist Jonathan Haidt argues that the decline of free-play in childhood and the rise of smartphone usage among adolescents are the twin sources of increased mental distress among teenagers. Haidt delves into the latest psychological and biological research to show how, between 2010 and 2015, childhood and adolescence got rewired. As teens traded in their flip phones for smartphones packed with social media apps, time online soared while time engaging face-to-face with friends and family plummeted, and so did mental health. This profound shift took place against a backdrop of diminishing childhood freedom, as parents over-supervised every aspect of their children`s lives offline, depriving them of the experiences they most need to become strong and self-governing adults.