The Great Rewiring: a specific, dateable shift in childhood

Haidt anchors the book in a striking pattern: rates of teen depression, anxiety, self-harm, and suicide, which had been relatively flat for years, began rising sharply and simultaneously across the U.S., U.K., Canada, Australia, and other countries starting around 2012 — and the timing lines up closely with the mass adoption of smartphones with front-facing cameras and the shift of social media from desktop to constant, mobile, algorithm-curated feeds.

He calls this transition the Great Rewiring of Childhood: a shift from a childhood organized around free, unsupervised, physical play to one organized around a phone-based existence, and argues the synchronized international timing is a crucial clue, since it rules out explanations specific to any one country's politics or economy.

Haidt is careful to argue correlation plus mechanism plus international consistency — not correlation alone — as his evidentiary case, since he anticipates the natural objection that correlation isn't causation, and builds the rest of the book around supplying the causal mechanisms.