Scientists believe they have discovered the trigger that turns children into troublesome teenagers.

At puberty, their response to a brain signalling chemical released by stress suddenly alters. Instead of reducing anxiety, it increases it, at least according to experiments in mice. In the distant past, such an abrupt change may have had a survival advantage. In many species, puberty is the time to leave home and make one’s way among strangers.

Faced with stressful circumstances, anxiety may be the right response to ensure survival. But in adolescents who have years to go before they leave home, the abrupt change in behaviour no longer serves any purpose except to exasperate their parents.

A team at the State University of New York Downstate Medical Centre in Brooklyn studied the behaviour of a steroid called tetrahydropreg-nanalone (THP). Produced in response to stress, THP normally acts to reduce anxiety. But in mice the process changes suddenly at puberty. Under the influence of sex hormones, the brain begins producing higher numbers of a different receptor, to which THP attaches itself. Its effects are then reversed: instead of causing the mice to become calm, THP causes them to become more excitable.

The team, led by Sheryl Smith, speculate that this change leads to mood swings that characterise the teenage years. The same effect, they say in Nature Neuroscience, may occur in premenstrual tension and postmenopausal irritability. However, so far the experiments have been conducted only using mice.

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