They have a reputation for being sulky, uncommunicative and argumentative little brats. But according to scientists, blaming teenagers for their tantrums and mood swings is - in the words of Kevin the teenager - "just SO unfair".

Their petulant behaviour is all down to an unexpected chemical reaction in their developing brains. Scientists have found that the mechanism normally used by the brain to calm itself down in stressful situations seems to work in the opposite way in teenagers, making them even more anxious.

When the brain senses a stressful situation, it reacts by switching on receptors, using a range of chemicals, including a steroid called THP. In an adult or even a younger individual, THP would reduce anxiety. But in experiments on adolescent mice, THP increased anxiety.

The experiments, by Sheryl Smith, a physiologist at the State University of New York, offer the first physiological explanation for adolescent mood swings. Previous work has focused on analysing behavioural changes in teenagers during adolescence. Her results are published today in Nature Neuroscience.

In teenagers, the behavioural response to the increased anxiety due to THP would result in even more acute stress, said Prof Smith. How individuals reacted would depend on their personality - where some people might cry, others would get angry.

"It could be an emotional reaction and it might be fluctuating too because it's an amplified reaction to the stress which may not seem the appropriate reaction to an adult. Yet, for the teenager, they don't have a whole lot of control. It could be frustrating for the teenager and the parent. This is a reaction that seems like an over-reaction to the adult, perhaps. But to the teenager, it is absolutely the only thing they are able to do."

Prof Smith said that the paradoxical effect in teenagers' brains can be explained by the fluctuation in hormones that accompanies this phase of their lives. She successfully tested this idea by artificially altering the levels of certain hormones in the brains of her mice.

The social awkwardness of teenagers has been the subject of several studies in recent years. Scientists have shown that the combination of having a brain that is not yet fully developed and the hormonal stresses of puberty can explain much of the infamous behaviour of adolescents.

Scientists at University College London presented research last year showing that teenagers could not help being sulky because they were still developing social skills and did not have the full mental hardware to see the world from someone else's point of view or predict the consequences of their actions.