Mystery of lazy teenager solved

Harry Enfield's Kevin and Perry characters struck a chord with parents everywhere, with their constant sulking and idleness.

However, new research has revealed that troublesome teens can't help being lazy.

Scans have shown that the part of the brain dealing with motivation was found to be far less active or developed in teens than in adults.

"They just seem to have a huge lack of drive to do anything," said Dr David Hummer of the US National Institute on Alcohol Abuse and Alcoholism, which carried out the study. "It seems that area of their brain isn't yet developed.

"We also discovered teens tend to take more risks. This could explain why they disobey their parents and why drinking and drug abuse is a problem - it just takes a lot more to excite a teen than an adult." Dr Hummer's team carried out brain scans on 12 teenagers and 12 people over 20 while they played a specially-designed gambling game, where they were given differing financial incentives to hit a target.

Although teens and adults rated the rewards equally when questioned and hit the targets with equal frequency, their brain scans showed a different story.

The part which regulates motivation - the right ventral striatum - was shown to be much less developed in the adolescents than in the adults. "The teens just weren't interested in long-term reward at all - everything was about short term gain," said Dr Hummer.

"They all said they were excited, but their brains showed a different picture."

His team hopes to conduct further experiments to establish the exact age at which the motivational area develops fully.

Phillip Hodson, of the British Association for Counselling and Psychotherapy, said: "This research would certainly back up what we see every day. However, I think it's important to point out that none of these studies say this phase is a bad thing. It's perfectly natural to rebel."