Don't give your kid a smartphone before high school, and don't let them use social media before age 16, New York University social psychologist Jonathan Haidt argues in a new book.

Why it matters: The shift from "play-based" to "phone-based" childhoods is making our kids sick and miserable, Haidt argues.

Driving the news: In "The Anxious Generation: How the Great Rewiring of Childhood Is Causing an Epidemic of Mental Illness," out March 26, Haidt says that staring at screens all the time is terrible for human development.

• A "phone-based" childhood causes "social deprivation, sleep deprivation, attention fragmentation and addiction," Haidt writes.
• A "play-based" childhood is essential for developing physical and social skills, like conflict resolution.

"Children learn through play to connect, synchronize, and take turns," the book says. "They enjoy attunement and need enormous quantities of it."

• Social media, by contrast, "is mostly asynchronous and performative," Haidt writes. "It inhibits attunement and leaves heavy users starving for social connection."

What they're saying: Haidt offers four controversial suggestions:

1. No smartphones for kids before high school — give them only flip phones in middle school.
2. No social media before age 16.
3. Make schools phone-free, by putting devices in phone lockers or Yondr pouches.
4. Give kids far more free play and independence, including more and better recess.

Zoom in: Banning phones in schools "is the easiest and fastest step we can take to improve youth mental health," Haidt writes.

• "We could, in theory, have all K-12 schools in the U.S. enact the policy for this coming September," Haidt wrote on Substack.
• "That would give all young people six or seven hours a day away from TikTok and texting, freeing up enormous amounts of time for learning from their teachers and developing friendships with other students."

Context: Since 2018, Haidt has been researching "the contributions of social media to the decline of teen mental health and the rise of political dysfunction," as his website puts it.

• He and other social scientists, like Jean Twenge of San Diego State University, link the dramatic rise in adolescent mental health problems to smartphones and social media.
• Legislators have put pressure on TikTok, Instagram, Facebook and others, but to little effect.

By the numbers: The "teen mental illness epidemic began around 2012," Haidt asserts, presenting ample research to back it up.

• The numbers started rising noticeably in 2010 — three years after the introduction of the iPhone.
• Rates of depression and anxiety among U.S. adolescents were "fairly stable in the 2000s" but "rose by more than 50% in many studies from 2010 to 2019," Haidt writes in The Atlantic.
• The suicide rate rose 48% for adolescents ages 10 to 19. For girls ages 10 to 14, it rose a staggering 131%.

The big picture: "The primary thing that we are trying to understand is why adolescent mental health fell off the cliff right around 2010," Zach Rausch, Haidt's research partner, tells Axios.

• "The core thesis that we make in the book is that we started overprotecting kids long before 2010 — it really began in the 1980s."
• "We started pulling kids indoors, giving them much more supervision in highly structured activities and much less independence, free play and responsibility."
• By 2010, "social life for adolescents in particular moved almost entirely onto smartphones and social media platforms, and completely away from this in-person, real-world childhood and adolescence."

Case study: An organization called "Let Grow" is trying to get schools and parents to give kids more independence and unstructured time.

• Haidt co-founded the group along with Lenore Skenazy — who coined the term "free-range parenting" and was once dubbed "America's worst mom" for letting her 9-year-old take the New York City subway alone.

The other side: Critics such as statistician Aaron Brown praise Haidt's integrity, but question his conclusions and methodology.

• Used in moderation, social media can help kids beat loneliness and enhance friendships, some experts say.

Reality check: Putting the cellphones-and-social-media genie back in the bottle for kids is going to be a tough sell.

• Parents are often the ones demanding to be able to reach their kids during the school day.
• They're also the ones pleading with their kids to put the phones down — without success.

The bottom line: The problems Haidt identifies are big and easy to recognize.

• "Teens and parents are caught in this social trap, and the only way to get out of it is through collective action," Rausch says.