Why do good kids make bad decisions when their friends are around? Turns out there’s a biological explanation for peer pressure. Not that it makes things like this any more comprehensible.)

Scientists at Temple University used MRIs to study the brain activity of 40 teenagers and adults playing a driving game in which participants had to decide whether to speed through yellow traffic lights in order to earn a higher score, even if meant crashing. Adults showed no difference in their playing when their friends were present, but the teens ran 40 percent more yellow lights and had 60 percent more crashes when they knew their friend were watching.

According to the New York Times Well blog:

“The presence of peers activated the reward circuitry in the brain of adolescents that it didn’t do in the case of adults,” said Laurence Steinberg, an author of the study, who is a psychology professor at Temple and author of “You and Your Adolescent: The Essential Guide for Ages 10 to 25.” “We think we’ve uncovered one very plausible explanation for why adolescents do a lot of stupid things with their friends that they wouldn’t do when they are by themselves.”

“All of us who have very good kids know they’ve done really dumb things when they’ve been with their friends,” Dr. Steinberg said. “The lesson is that if you have a kid whom you think of as very mature and able to exercise good judgment, based on your observations when he or she is alone or with you, that doesn’t necessarily generalize to how he or she will behave in a group of friends without adults around. Parents should be aware of that.”

What this means for parents is that we need to supervise our teenagers very closely. As we’ve seen, good judgment can be as much a matter of biological development as it is a personality trait.