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?
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?
Snippet courtesy of Coligny flipping Snowden the bird: BBC News