About 10 years ago, Juvonen set out to capture how peer relationships change over the course of adolescence. Over a period of three years, she and her team recruited 6,000 sixth graders from 26 different middle schools in Los Angeles and then followed each cohort. Every year, the participating children filled out a series of questions about peers: Name your closest friends. Does this kid have your back? Can you talk to him or her about anything? Do they come to your house? Have you ever been bullied? Have you seen anyone else be bullied?

The study revealed that instability rules, at least at the beginning. Two-thirds of the children entering their first year of middle school changed friends between the fall and the spring. Juvonen suspects that has to do with the structure of the school system. Students arrive from smaller elementary schools knowing a few other children from fifth grade. At the start of the year, they stay close physically and emotionally to those familiar classmates. But as they settle into life in the new environment, their social horizons expand. They gravitate to those with similar interests of the kind that begin to solidify in these years—soccer, theater, robotics. Similarities, as always, attract. Earlier friends often fall by the wayside.

Friendship has real power for kids. Juvonen thinks that friendship may even begin to resemble an attachment relationship like what children initially have with parents. “[These] are really very, very close and emotionally intimate relationships,” Juvonen told me. “And even if that particular relationship doesn’t last, it has ramifications on ­subsequent relationships.”