Everyone Says Social Media Is Bad for Teens. Proving It Is Another Thing.

There have been increasingly loud public warnings that social media is harming teenagers’ mental health — most recently from the United States surgeon general — adding to many parents’ fears about what all the time spent on phones is doing to their children’s brains.

While many scientists share the concern, there is little research to prove that social media is harmful — or to indicate which sites, apps or features are problematic. There isn’t even a shared definition of what social media is. It leaves parents, policymakers and other adults in teenagers’ lives without clear guidance on what to be worried about.

“We have some evidence to guide us, but this is a scenario where we just need to know more,” said Jacqueline Nesi, a psychologist at Brown who studies the topic.

What counts as social media when it comes to teenagers’ health?

The surgeon general, Dr. Vivek Murthy, warned last month that social media carried a “profound risk of harm,” but he didn’t name any apps or websites. His report acknowledged that “there isn’t a single, widely accepted scholarly definition of social media.”

Most studies look at platforms with user-generated content, where people can interact. But that raises a lot of questions. Does it matter if teenagers see posts from people they know or don’t know? Does it make a difference if they post or just view? Do multiplayer games count? Dating apps? Group texts?

YouTube illustrates the challenge. It’s the most popular site among teenagers by far: 95 percent use it, and almost 20 percent say they do so “almost constantly,” Pew Research Center found. It has all the features of social media, yet it hasn’t been included in most studies.