Haidt opens with the book's central historical claim: sometime around 2010–2012, childhood in wealthy countries shifted from being organized around free, unsupervised play to being organized around smartphones and social media — and this shift, not some other cultural factor, is the primary driver of the teen mental health crisis that began at exactly that time.

Key evidence he presents:

• Rates of teen anxiety, depression, self-harm, and suicide in the US and other countries begin a sharp, synchronized rise starting around 2012 — closely tracking smartphone and social media adoption among teens, not other candidate causes like the 2008 recession or general modern stress
• The decline is disproportionately severe for girls, which Haidt links to social media's particular emphasis on appearance, comparison, and relational aggression
• The correlation appears consistently across many countries with different school systems, economies, and cultures, which Haidt argues points to a common technological cause rather than a local one

This opening chapter sets the book's core diagnostic claim that the rest of the book builds its case and solutions around.