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As many as a million young people in Japan are thought to remain holed up in their homes - sometimes for decades at a time. Why?

For Hide, the problems started when he gave up school.

"I started to blame myself and my parents also blamed me for not going to school. The pressure started to build up," he says.

"Then, gradually, I became afraid to go out and fearful of meeting people. And then I couldn't get out of my house."

Gradually, Hide relinquished all communication with friends and eventually, his parents. To avoid seeing them he slept through the day and sat up all night, watching TV.

"I had all kinds of negative emotions inside me," he says. "The desire to go outside, anger towards society and my parents, sadness about having this condition, fear about what would happen in the future, and jealousy towards the people who were leading normal lives."

Hide had become "withdrawn" or hikikomori.

In Japan, hikikomori, a term that's also used to describe the young people who withdraw, is a word that everyone knows.

Tamaki Saito was a newly qualified psychiatrist when, in the early 1990s, he was struck by the number of parents who sought his help with children who had quit school and hidden themselves away for months and sometimes years at a time. These young people were often from middle-class families, they were almost always male, and the average age for their withdrawal was 15.

It might sound like straightforward teenage laziness. Why not stay in your room while your parents wait on you? But Saito says sufferers are paralysed by profound social fears.

"They are tormented in the mind," he says. "They want to go out in the world, they want to make friends or lovers, but they can't."

Symptoms vary between patients. For some, violent outbursts alternate with infantile behaviour such as pawing at the mother's body. Other patients might be obsessive, paranoid and depressed.

Otaku v hikikomori Image copyright Getty Images An overlapping group of people with the hikikomori, otaku are "geeks" or "nerds"

are "geeks" or "nerds" They are known for their obsessions, especially manga cartoons and anime

"Otaku" is the formal word for "you" in Japanese - it's thought that the term came about from the tendency of socially awkward manga fans to use over-formal language

In press coverage, both otaku and hikikomori have been linked with serious sex crimes

When Saito began his research, social withdrawal was not unknown, but it was treated by doctors as a symptom of other underlying problems rather than a pattern of behaviour requiring special treatment.

Since he drew attention to the phenomenon, it is thought the numbers of hikikomori have increased. A conservative estimate of the number of people now affected is 200,000, but a 2010 survey for the Japanese Cabinet Office came back with a much higher figure - 700,000. Since sufferers are by definition hidden away, Saito himself places the figure higher still, at around one million.

The average age of hikikomori also seems to have risen over the last two decades. Before it was 21 - now it is 32.

So why do they withdraw?

The trigger for a boy retreating to his bedroom might be comparatively slight - poor grades or a broken heart, for example - but the withdrawal itself can become a source of trauma. And powerful social forces can conspire to keep him there.

One such force is sekentei, a person's reputation in the community and the pressure he or she feels to impress others. The longer hikikomori remain apart from society, the more aware they become of their social failure. They lose whatever self-esteem and confidence they had and the prospect of leaving home becomes ever more terrifying.

Parents are also conscious of their social standing and frequently wait for months before seeking professional help.

Image copyright Kadokawa Shoten Image caption "I don't want to talk to anybody. I don't want to do anything. I don't even have the will to pick up the phone. Just what am I supposed to do?" Welcome to NHK! was a novel, comic book and cartoon about the life of a hikikomori. (Copyright Tatsuhiko TAKIMOTO 2004, Kendi OIWA 2004. Published by KADOKAWASHOTEN.)

A second social factor is the amae - dependence - that characterises Japanese family relationships. Young women traditionally live with their parents until marriage, and men may never move out of the family home. Even though about half of hikikomori are violent towards their parents, for most families it would be unthinkable to throw them out.

But in exchange for decades of support for their children, parents expect them to show respect and fulfil their role in society of getting a job.

Matsu became hikikomori after he fell out with his parents about his career and university course.

What about the girls? Image copyright Thinkstock Hikikomori are seen as predominantly male - Tamaki Saito says males occupy 70%- 80% of the group

However, an internet survey by NHK found just 53% to be male

Female withdrawal into the home seems so natural to Japanese society that it may remain unreported, speculates Andy Furlong at the University of Glasgow

"I was very well mentally, but my parents pushed me the way I didn't want to go," he says. "My father is an artist and he runs his own business - he wanted me to do the same." But Matsu wanted to become a computer programmer in a large firm - one of corporate Japan's army of "salarymen".

"But my father said: 'In the future there won't be a society like that.' He said: 'Don't become a salaryman.'"

Like many hikikomori, Matsu was the eldest son and felt the full weight of parental expectation. He grew furious when he saw his younger brother doing what he wanted. "I became violent and had to live separately from my family," he says.

One way to interpret Matsu's story is see him as being at the faultline of a cultural shift in Japan.

"Traditionally, Japanese psychology was thought to be group-oriented - Japanese people do not want to stand out in a group," says Yuriko Suzuki, a psychologist at the National Institute for Mental Health in Tokyo. "But I think especially for the younger generation, they want more individualised or personalised care and attention. I think we are in a mixed state."

But even hikikomori who desperately want to fulfil their parents' plans for them may find themselves frustrated.

Andy Furlong, an academic at the University of Glasgow specialising in the transition from education to work, connects the growth of the hikikomori phenomenon with the popping of the 1980s "bubble economy" and the onset of Japan's recession of the 1990s.

It was at this point that the conveyor belt of good school grades leading to good university places leading to jobs-for-life broke down. A generation of Japanese were faced with the insecurity of short-term, part-time work.

And it came with stigma, not sympathy.

Job-hopping Japanese were called "freeters" - a combination of the word "freelance" and the German word for "worker", arbeiter. In political discussion, freeters were frequently bundled together with "neets" - an adopted British acronym meaning "not in education, employment or training". Neets, freeters, hikikomori - these were ways of describing the good-for-nothing younger generation, parasites on the flagging Japanese economy. The older generation, who graduated and slotted into steady careers in the 1960s and 1970s, could not relate to them.

"The opportunities have changed fundamentally," says Furlong. "I don't think the families always know how to handle that."

Image copyright Getty Images Image caption University graduates at a job-hunting fair in February... but freeters, neets and hikikomori find themselves on the periphery of Japan's labour market

A common reaction is for parents to treat their recalcitrant son with anger, to lecture them and make them feel guilty for bringing shame on the family. The risk here is that - as with Hide - communication with parents may break down altogether. But some parents have been driven to extreme measures.

For a time one company operating in Nagoya could be hired by parents to burst into their children's rooms, give them a big dressing down, and forcibly drag them away to a dormitory to learn the error of their ways.

Kazuhiko Saito, the director of the psychiatry department at Kohnodai Hospital in Chiba, says that sudden interventions - even by healthcare professionals - can prove disastrous.

"In many cases, the patient becomes violent towards the staff or the parents in front of the counsellors, or after the counsellors have left," he says.

Kazuhiko Saito is in favour of healthcare professionals visiting hikikomori, but he says they must be fully briefed on the patient, who must know in advance that they are coming.

Hikikomori - just a Japanese thing? Image copyright Thinkstock Hikikomori has entered the Oxford English Dictionary as "In Japan: abnormal avoidance of social contact"

But Saito Tamaki believes it is also a problem in Korea and Italy

After a 2002 BBC documentary, Saito received a flurry of emails from British parents who said their children were in a similar condition

Andy Furlong points out that young people in Western societies frequently "take time out" in gap years or have "false starts" on careers or courses without attracting stigma

He adds that the preconditions for a hikikomori-like problem are falling into place in Europe, with 50% youth unemployment in some countries, forcing young people to continue living at home

In any case, the do-nothing approach has been shown not to work. Tamaki Saito likens the hikikomori state to alcoholism, in that it is impossible to give up without a support network.

His approach is to begin with "reorganising" the relationship between the patient and his parents, arming desperate mothers and fathers with strategies to restart communication with their children. When the patient is well enough to come to the clinic in person he can be treated with drugs and therapy. Group therapy is a relatively new concept to Japanese psychology, but self-help groups have become a key way of drawing hikikomori into wider society.

For both Hide and Matsu, the journey to recovery was helped by visiting a charity-run youth club in Tokyo known as an ibasho - a safe place for visitors to start reintroducing themselves to society.

Both men have made progress in their relationships with their parents. Matsu has been for a job interview as a computer programmer, and Hide has a part-time job. He thinks that by starting to talk again with his parents, the whole family has been able to move on.

"They thought about their way of life in the past and in the future," he says. "I think that before - even though they were out working - their mental attitude was just like a hikikomori, but now they're more open and honest with themselves. So as their child I'm very happy to see them change."

Many parents of hikikomori visit the ibasho even though their children may never be well enough to come with them.

Yoshiko's son withdrew from society very gradually when he was 22.

At first he would go out to buy shopping, but she observes ruefully that internet shopping means this is no longer necessary and he no longer leaves the house. He is now 50 years old.

"I think my son is losing the power or desire to do what he wants to do," she says. "Maybe he used to have something he wanted to do but I think I ruined it."

You can listen to The Truth about Mental Health: Hikikomori on the BBC World Service at 14:30 GMT on Friday 05 July.

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Readers shared their stories about refusing to leave the bedroom.