Japan's health ministry defines hikikomori as a lifestyle in which a person "does not go to work or school, rarely interacts with people other than family members, and has been housebound consecutively for six months or longer."
Hikikomori is listed in the Oxford English Dictionary as "the abnormal avoidance of social contact, typically by adolescent males."
A survey conducted by Japan's Cabinet Office four years ago found that an estimated 541,000 people between 15 and 39 were hikikomori.
But a similar Cabinet Office survey carried out last December found that an estimated 613,000 people between 40 and 64 also fell into the category. It was the first time this age range was surveyed, and the number surpassed the 15 to 39 range that hikikomori are typically thought to fall in.
Yamase has held a number of jobs since dropping out of school, but he says he couldn't fit in. He always quit and went back home. He says he has been a hikikomori for more than 10 years in total.
Yamase's father died nine years ago, and now he lives at home with his 87-year-old mother. He has a developmental disorder and struggles with everyday tasks. He depends on his mother for all of his needs, like meals, laundry, and shopping.
The two get by on their savings and money his mother earns from renting out properties. Yamase says he wants to find work and contribute. He goes to a support center that helps people with disabilities find work, but he says his prospects are dim.
Yamase says he feels bad about being a hikikomori and making his elderly mother look after him. He admits that he's lucky his mother is still in good health. He says he would probably be unable to look after her if she needed care, or handle the necessary procedures if she died. He says thinking about the future makes him anxious.
The Cabinet Office survey found that 75 percent of hikikomori between 40 and 64 are men. About 21 percent say they have lived this way for 3 to 5 years. Overall, more than 50 percent say they have been Hikikomori for 5 years or longer. Some have even lived this way for 30 years or longer.
The survey also asked who the main income provider was in hikikomori households. 34.1% said "both parents."
These cases are referred to as the "8050 problem." The name comes from the situation in which parents in their 80s support hikikomori children in their 50s. It is becoming increasingly prevalent in Japan.
As people remain hikikomori for longer, their parents are growing older, forced to rely on small incomes and pensions to support them. This has led to hikikomori households becoming further isolated from society, living in increasing poverty.
Morito Ishizaki is editor-in-chief of "HIKIPOS" magazine. It publishes stories about the experiences and thoughts of hikikomori.
"Young hikikomori still believe they might be able to reintegrate themselves into society," he says. "But once people reach their 50s, they lose hope."
Ishizaki also says while young people worry about friends and interpersonal relationships, for middle-aged and older people, just being able to survive becomes the most pressing issue."
He says one reason the number of older hikikomori has surpassed the number of younger ones is that a lot of people who joined the workforce during Japan's period of high economic growth have quit and remained jobless. He thinks these people are unable to find a purpose in life other than work and have become "socially isolated" at home as a result.
Ishizaki adds that "social isolation" and hikikomori should be treated as separate issues. He says many people he interacts with on a daily basis become hikikomori due to changes in their family environment and personal relationships. He says once someone becomes a hikikomori, the idea of leaving home makes them increasingly anxious.
In a statement on the recent survey, the Cabinet Office said "the condition of hikikomori is apparently affecting not only young people, but more middle-aged people than we imagined. We intend to come up with effective measures to deal with the situation including plans to expand support for those affected."
Ishizaki says it's only natural for people to find it difficult to interact with strangers and form deep relationships once they become older. He believes one of the most effective strategies may be to start supporting the parents of hikikomori. He says helping the parents would be a way to extend support to hikikomori without making them feel bad and burdening them mentally.