Unprotected sex, wild rides on a motorbike, smoking, drugs or alcohol -- the litany of risk-taking by teenagers is famously long. But what drives it? The answer could lie with adolescents' hypersensitivity to signals that unleash dopamine, a powerful brain chemical that underpins the pleasure from a reward, US psychologists suggest. A team led by Jessica Cohen of the University of California, Los Angeles, recruited 45 people from three age groups -- children aged eight to 12, teenagers aged 14-19 and adults aged 25-30. The volunteers were asked to look at pictures on a computer screen and say whether the images matched the pattern on T-shirts sold by either of two fictitious universities. There was a financial carrot (either 25 US cents or five cents) for each answer that was right. The answer could be either predictable or random. The task was performed while the volunteers were in a functional magnetic resonance imaging scanner (fMRI), which measures blood flow in the brain in response to neural signals. Among the teenagers, the striatal area of the brain, which is sensitive to dopamine, lit up more strongly than among children and adults when receiving the reward. 'Our research shows that when adolescents get a reward that they're not expecting, their brains are more responsive to that reward,' Cohen said in a phone interview. Dopamine levels were not measured, but 'it's our guess that dopamine is the cause,' Cohen said. Previous research, among adult humans and monkeys, has found dopamine surges before an expected reward comes, she noted. The findings, published by the journal Nature Neuroscience, could help parents, school teachers and others guide adolescents in the transition from childhood to adulthood, says Cohen. Children are not fully sensitive to rewards, whereas adults are sensitive to rewards but also -- to varying degrees -- brake the urge. 'Some researchers have put forward a theory that striatal regions are fully developed in adolescents but the pre-frontal regions, which put the brakes on, are not,' Cohen explained. 'As a result, adolescents get the sensitivity to reward that adults get too. But adults can suppress it and think before they act, or act more responsibly sometimes, whereas teenagers tend not to be able to do this as well.' 