Drivers did not make their decisions entirely alone. Sometimes the friends they had brought were in the room with them. Sometimes the friends were in the next room visible on a monitor but unable to communicate with the driver. The results were striking. With friends in the room watching, adolescents regularly took more chances. Adults did not. With friends out of the room but nearby, watching on a monitor but unable to communicate, adolescents still took more chances. In that situation, it wasn’t possible for the friends to exert verbal peer pressure, but it didn’t matter. “When teenagers knew their friends could see their performance, it increased the amount of risk taking they engaged in compared to when they were alone,” Steinberg told me.

Then Steinberg joined forces with the Temple neuroscientist Jason Chein and began running the same experiments with the “driver” in a brain scanner. They saw the same peer effect, and now they could see what was going on in the brain as well. “When kids were in the presence of peers, it activated reward centers in the brain,” Steinberg said. “The more that happened, the more risks kids took.” The scientists developed a more nuanced theory than one about pressure. “When kids are around other kids it primes their reward system to be more easily aroused and more easily activated. That in turn leads them to pay undue attention to the potential rewards of a risky choice and relatively less to the potential costs.”

Well, okay, but what if just knowing the friends are there is still a form of peer pressure? The teenager being tested no doubt suspects that what would impress his friends is to race through the intersections and finish in record time. In anticipation of this, Steinberg, Chein, and their colleagues came up with a way to rule out that possibility. They needed adolescents who wouldn’t or couldn’t care what their friends thought of them. They used mice.

After raising peer groups of mice, Steinberg and Chein gave them alcohol, which triggers reward systems in mouse brains just as it does in human brains. They randomly assigned the mice to be tested alone or in the presence of their peers, and tested half as juveniles (the equivalent of adolescents) and half as adults. In the presence of other mice, adolescent mice drank more than they did when they were alone. In adults, there was no difference in the amount that they drank. “There’s something about the brain during adolescence in mammals that is hardwired to be especially sensitive to peer influence and to be more reward-seeking in the presence of peers,” Steinberg said. Instead of calling the phenomenon peer pressure, they began calling it “peer presence.”

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Importantly, peer presence can be a force for good as well as for bad. “When teenagers are with each other, everything that feels good feels even better,” Steinberg said. If what feels good is something that also carries some danger to it, then kids get into trouble because they are ignorant of the danger—or choose to ignore it. But Steinberg and his colleagues have also shown that teenagers learn faster when they’re with their peers than they do by themselves. And they engage in more exploratory behavior when they’re with their peers.