“Why do otherwise good kids seem to make bad decisions when they are with their friends?” This opening question of a piece on the New York Times Well blog might have you thinking: How often would really good kids find themselves in that situation? Once in a while, yes, but habitually? Wouldn’t there be some lack in upbringing there, making them susceptible to peer pressure and choosing the wrong friends?

The biological argument, however, is that teenage brains are driving this phenomenon. In a laboratory experiment carried out by psychologists at Temple University in the US it turned out that, while college students and young adults were not significantly influenced by peers, 14- to 18-year-olds were. In a video driving game they ran about 40 per cent more yellow lights and had 60 per cent more crashes when they knew their friends were watching -- from an adjoining room. Brain scans showed the “reward centres” lighting up.

“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.”

Teenagers and their brains are a hot topic for research and it is now a truism of developmental science that the brain does not reach its adult form until about age 25. A report published in New Zealand today urges the government to raise the legal drinking age again to 21 (currently it is 18) -- and urges voluntary abstention until age 24-and-a-half -- on the ground that alcohol can have long-lasting ill effects on the immature brain.

Add alcohol to other risk-taking impulses and the dangers increase.

Dr Steinberg points out to parents that peer pressure doesn’t have to be direct; just the awareness that a friend is watching is enough. He also notes that “the brain system involved in reward processing is also involved in the processing of social information" -- to which adolescents are specially sensitive. The moral for parents seems to be that groups of teenagers need close supervision.

“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.”

Another thought: given friends with good values and upbringing, these biological mechanisms should reinforce good decision making too. Is there any reason why risk-taking should always be a negative thing? If a 15-year-old wants to take the risk of acting well in a difficult situation -- say, defending someone who is being bullied or gossiped about -- the fact that a friend who would approve is present should help them act.

Upbringing surely is more powerful than biology.