Rising expectations encouraged by education, the increasing autonomy and isolation of adolescents and high levels of family discord are all cited as possible factors in a trend which affects young people, but not adults, in Europe and North America.

“We have a major problem,” said one of the authors, Professor Sir Michael Rutter, head of child and adolescent psychiatry at London University’s Institute of Psychiatry. “We need to find answers.” He called on the Government to give a higher priority to research into the effects of youth policy.

However the study, published in book form as Psychosocial disorders in young people: time trends and their causes, fights shy of identifying the causes of the rise, and in fact concludes that some of the most popular theories are wrong.

While poverty is a factor in many individual cases, says the book, it does not appear to be the most influential one in the long term: the incidence of disorder was low in the depression of the 1930s and started to rise in the 1950s - a period of unprecedented affluence. The mass media are probably not to blame, even though they may magnify the effects of social change.

Nor did the authors, Professor Rutter and Professor David Smith, a criminologist from the University of Edinburgh, find any evidence of the moral decline of popular belief.

Their studies compared international data for trends in crime, alcohol and drug abuse, depression, eating disorders like anorexia nervosa, and suicide. They found that there had been a marked increase in all these categories in Europe and North America but not in Japan - a point which Professor Rutter said merited further study.

“Most people think it’s connected with the very much stronger social networks and the very much stronger kind of restraint within communities,” said Professor Smith.

Launching the report, the authors suggested that the development of a separate, youth culture could be among the factors to blame.

“Young people are more detached, separate from society,” said Professor Rutter. “They have become in some respects a separate class.”

He expressed disappointment that they had been unable to arrive at firmer conclusions. “Both of us find it very frustrating. It would seem that something as striking as this ought to have an equally striking explanation.”

The report lists 10 main conclusions: * Worsening living conditions do not directly account for the rising levels of disorder although poor social circumstances may increase other risk factors.

* Nor is increasing rise in affluence to blame, although it is likely to increase opportunities for crime and drug abuse.

* Unemployment does not explain the rise in disorder since the Second World War.

* Worsening health does not account for the rise, although poor physical health creates mental hazards.

* Increasing levels of family discord and break-up may well have played a role. However, the evidence indicates that the main risk stems from discord and lack of parental support rather than broken families in themselves.

* The changing patterns of transitions in adolescence and early adult life may cause risks associated for example with a growth of youth culture, a possible increasing isolation from adults, earlier engagement in sexual relationships, an increase in peer-group influence and a greater number of breakdowns in cohabiting love relationships.

* It is most unlikely that adverse effects of mass media account for the rise in disorder. However they may augment the effect of social change.

* There have been changes in moral values but no evidence of a general moral decline.

* It is possible that the increase in people’s expectations with a parallel difficulty in meeting them has played some role.

* The evidence suggests that the explanations are different for different specific disorders.

Psychosocial disorders in young people by Michael Rutter and David J Smith is published this week by John Wiley Sons on behalf of Academia Europaea. ISBN 0-471-95054-8, price Pounds 49.95.