The biggest cry for help is coming from that group. Fortunately, it’s a smaller number than victims overall. Bully-victims, Copeland and others have found, have more problems at home and the most trouble with impulse control and aggression. Sometimes they do the dirty work for popular kids who bully to curry favor with them. “I don’t think things are working out socially for them in a lot of ways,” Copeland said.

It’s important to point out that Copeland and other researchers don’t define bullying broadly, in a way that encompasses a lot of mutual conflict among kids, or one-time fighting. Bullying is physical or verbal harassment that takes place repeatedly and involves a power imbalance — one kid, or group of kids, making another kid miserable by lording power over him.

As Dan Olweus, the Scandinavian psychologist who launched the field of bullying studies in the 1960s, has been arguing for many years, this is a particular form of harmful aggression. And so the effort to prevent bullying isn’t about pretending that kids will always be nice to each other, or that they don’t have to learn to weather some adversity.

If the results of this study are dismaying because they indicate that bullying is permanently scarring, the findings also strengthen the argument for prevention. Copeland underscores this idea. “Consider me a reluctant convert, but I’m starting to view bullying the same way I do abuse in the home,” he said. “I honestly think the effects we’re observing here are just as potent.”

Emily Bazelon is a Slate senior editor and writes about law, family, and kids.