Teenagers do crazy things, and the chemistry of their brains might explain why, say researchers.
The adolescent brain is extra sensitive to reward signals when pay-off for a risk is higher than expected, say cognitive neuroscientist Dr Russell Poldrack, from the University of Texas, Austin and colleagues.
They say the discovery might help explain why teens take risks that don't seem worth it to adults - from driving too fast to experimenting with drugs.
"Teenagers seek out these sorts of rewarding experiences, and this provides a little explanation for that," says Poldrack, whose research is published in this week's Nature Neurosciece.
"In the long run, it may help us understand how addictions start and develop."
To zero in on the neuroscience behind risk-taking behaviour in adolescents, Poldrack and colleagues focused on a concept called prediction error, which describes the difference between what a person expects to happen and what actually happens.
If you anticipate a rich sip of full-bodied espresso, for example, but you end up gulping weak, watery, and burnt coffee, that's a negative prediction error.
If you expect nothing from a friend for your birthday but he gives you $20, that's a positive error - far better than expected.
To test the brain's reaction to positive prediction errors at different stages of life, the scientists enlisted 45 people, ranging in age from 8 to 30.
Each participant was shown a series of abstract kaleidoscopic images and challenged to categorise the figures as logos belonging to one of two fictional colleges.
When they got an answer right, participants earned a small amount of money and they all gradually learned which logos went with which colleges and which ones were worth more than others.
There were a few twists, though: Sometimes, an answer that should've been correct was judged as wrong.
Sometimes, a wrong answer was rewarded as a correct one. And sometimes, the reward was larger or smaller than the exercise indicated it should be.
With a mathematical model, the researchers were able to determine how much money each person expected to get with each answer and compare that with what they actually received. At the same time, fMRI's showed what was happening in the brain.
Previous research has shown a surge of activity in a brain region called the ventral striatum when reality exceeds a person's expectations.
In the new study, the region's response was highest in participants between 14 and 19 years old when they received more money than anticipated.
Brain activity in the ventral striatum is related to the release of dopamine, a nerve-signalling molecule that helps the brain process rewards and can be involved in addictions.
With more dopamine flowing, a teenager is likely to feel that a risky behaviour - when it ends well - is so much more rewarding than it might seem to a child or adult.
So, for example, the social rewards of staying out past curfew might outweigh the likelihood of getting in trouble for an adolescent.
And the physical pleasure of getting drunk might outweigh the dangers, including the next day's hangover.
Besides providing insight into how addictions might begin in adolescence, the new study might help parents channel their teens into more positive risk-taking activities, like playing sports or acting in school plays, suggests Dr Adriana Galvan, a developmental cognitive neuroscientist at the University of California, Los Angeles.
"Adolescents are uniquely sensitive to the uncertainty in the world," says Galvan. "Perhaps their willingness to engage in uncertainty is driven by the potential rewards that might result from that uncertainty. For them, the rewards loom so much bigger than the potential negatives."