HAMBURG (Deutsche Presse-Agentur (dpa)) -- Outside the sun is shining, but behind the closed blinds it's dark, just the way the girl inside feels. At 15 she joined a group of teenagers in similar difficulties in the German city of Hamburg.

By then she was fully in the grip of depression, but Joerg Lohgard is certain the illness struck much earlier in her life, although he does not know when. The social worker looks after a group of youngsters who have a particularly difficult history or are facing serious psychological problems.

More than 10 per cent of boys and twice that proportion of girls up to the age of 18 are believed to have suffered depression at some stage in their young lives. The figure suggests that virtually every classroom has someone suffering from depression.

Depression is thus one of the most frequent psychological ailments. The World Federation for Mental Health has begun to see a global crisis in the way depression is rising.

Nevertheless, half of all cases are not recognized, according to child psychotherapist Martin Baierl. He attributes this to parents with children with difficulties seek help from their doctors.

But parents should consider approaching a psychologist or therapist specializing in children if they see a child no longer laughing, showing few signs of enjoying themselves or not taking pleasure in their hobbies, Baierl says.

The signs of depression are little different from those in adults. Michael Schulte-Markwort, who works at a Hamburg university clinic, lists these as perpetual sadness, lack of drive, sleeping and eating problems among others.

Inability to concentrate, aggression or lack of self-worth could also be symptoms. "Depression has many non-specific symptoms," Baierl says. "A doctor qualified in the area has to make the final diagnosis."

The condition is often caused by external factors that were particularly devastating. "The most frequent cause is the separation of the parents," Schulte-Markwort says. In fact, any lasting psychological burden or even a serious problem of short duration can be the cause.

These could include feeling overworked at school, conflict with parents or feeling out of place in the community. Lohgard says his patient saw her father descend into alcoholism, her parents separating and her mother failing to cope as a single parent with three children. The girl began to experience a sensation of loss and a lack of self-worth.

Children need help just as much as adults to get over the effects of depression. Once diagnosed, the treatment usual consists of talking about the problems with an expert or behavioural therapy.

When the symptoms are more severe, this treatment is supported with the aid of antidepressants, but the most important aspect of the treatment is to establish and maintain good relations with the parents.

Just as certain life factors can cause or promote depression, others can have a preventive effect. A good home atmosphere, clear boundaries, a structured daily routine, encouragement from the parents and recognition tend to strengthen children and boost their psychological immune system.

"Offer praise to your child," Baierl says. "And solve any problems together. And parents must ensure that they are also feeling good about themselves, otherwise they are unable to help their children."

Copyright 2012 dpa Deutsche Presse-Agentur GmbH