Haidt, who previously explicated many of his thoughts about Gen Z in the New York Times bestseller The Anxious Generation, used a powerful metaphor to explain the neurological consequences of this change: tree roots. Saying they are great metaphors for neurons, Haidt explained that tree-root growth is structured by the environment where they are found. Referring to a picture of a tree growing around a Civil War-era tombstone, where the tombstone scratched the bark 100 years ago, and the tree adapted. The same is true for Gen Z, he argued: “Their brains have been growing around their phones very much in the way that this tree grew around this tombstone.”

Beyond mental health, Haidt said this has physical manifestations. Children are “growing hunched around their phone,” he said, with phone addiction literally “warping eyeballs,” leading to a global rise in myopia (short-sightedness). Screen time is also known to harm sleep, he added. He went on to describe a “great rewiring” of humanity, brought on by the smartphone.

A catastrophe of mental and physical health

This “great rewiring,” which Haidt places between 2010 and 2015, coincides with a synchronized global collapse in teen mental health. Haidt noted Gen Z is “suddenly much more mentally ill than the millennials,” primarily suffering from anxiety and depression.

The evidence of decline is seen in objective behavior, not just self-report. For instance, data tracking non-fatal self-harm for early teens (10 to 14 year olds) shows the girls’ rate “more than quintuples” between 2010 and 2015. Across the world, wherever the internet is in kids’ pockets, Haidt argued, young people are becoming less happy and less flourishing.

The transition Haidt describes occurred in two acts. Act One involved the gradual decline of the play-based childhood, which began in the 1980s. Act Two was the arrival of phone-based childhood, a sudden and universal shift that started in the early 2010s. Haidt summarized the tragic change by saying, “We have overprotected our children in the real world and we have underprotected them online.”

The erosion of focus and meaning

The crisis extends into cognitive ability. Haidt points out “50 years of progress ended in 2012” in educational achievement metrics, specifically the National Assessment of Education Progress, or NAEP, also called the “nation’s report card.” This decline suggests a “broader erosion in the human capacity for mental focus and application,” leading to what Haidt calls a “complete disaster for humanity”: a loss of that capacity. “We’re getting dumber exactly as our machines are getting smarter and taking over more areas of life,” he said.

Students themselves acknowledge the cognitive shift, according to Haidt. He related an anecdote from one of his students, describing the difficulty of reading: “I open a book, I read a sentence, I get bored, I go to TikTok.” Furthermore, he said high school seniors increasingly report “life often feels meaningless.” Haidt connected this directly to the time spent online, adding he can’t fully disagree: “if you’re spending five hours a day on social media, you’re not doing anything. Your life actually is meaningless.”

The paths to this “pit of despair” differ by gender. For girls, social media remains the “clearest culprit,” altering development, social relationships, and moods. For boys, the danger centers on a dopamine addiction crisis, with companies competing to “hook them” via highly addictive video games and increasingly available high-definition porn.

Haidt’s comments came as part of a symposium organized by Dartmouth economics professor David Blanchflower, whose work has previously been covered in Fortune. Most recently, he and University College London’s Alex Bryson found the midlife crisis has become a thing of the past, with a quarterlife crisis very real in reams of economic data. Young workers really are full of rising despair, their research found. Blanchflower told Fortune in September he’s “freaked” out by what his research is showing: “Suddenly young workers look to be in big trouble … Now, both absolutely and relatively, the young are worse off.” The midlife hump in despair, commonly known as the midlife crisis, used to be one of social science’s most important patterns, he added, and that’s over now.

The symposium occurred just weeks after an authority no less than Jerome Powell, chairman of the Federal Reserve, acknowledged Gen Z is having an especially hard time in the economy of 2025. “Kids coming out of college and younger people, minorities, are having a hard time finding jobs,” Powell said in mid-September, at his press conference following the Federal Open Market Committee meeting.

The solution: collective action

Haidt asserted the theory suggesting the rewiring of childhood is the only one that can handle the synchronized collapse in mental health globally. Given that this is a collective action problem, the solution must also be collective action, he argues.

Haidt proposed four key norms to reverse the phone-based childhood and restore the play-based model:

Delay Smartphones: Only give a flip phone or simple phone until high school or age 14 internationally.Social Media Age Limit: “No social media before 16.” Haidt stresses “we are completely insane if we give puberty over to social [media]”.Phone-Free Schools: Implement “bell-to-bell” policies, which teachers have welcomed, and studies are already showing raised grades.Promote Independence and Play: Encourage “far more independence, free play, and responsibility in the real world”.

Haidt stressed that although there will be a “permanent echo of diminished potential” in the generation that has already passed through puberty with these devices, “it’s not too late for individuals if they make an effort and they make it collectively.”

For this story, Fortune used generative AI to help with an initial draft. An editor verified the accuracy of the information before publishing.