– Japan is grappling with a mysterious problem among young people. Some 500,000 to 2 million are affected by a condition that leaves them unable to leave the house—or even their rooms—for years at a time. It's called hikikomori, a word that has been commonplace in Japan since the 1990s, though a few cases have also been seen in the US and other countries. "If you ask people in Japan about hikikomori, almost everyone will say, 'I know somebody like that,'" a psychologist said in the Japan Times last year. It's considered by many to be one of the country's top health issues, but what is going on, and whether those considered to be hikikomori are living with a mental illness, remains unclear, the Wall Street Journal reports. Treatment of the condition, which often affects young men, has been elusive. Every prefecture is legally required to have a treatment center, but few hikikomori actually visit them, and even when they do, few receive successful care. Now, international researchers are trying to understand the condition in the city of Fukuoka. As it stands, just half of hikikomori would likely receive a psychiatric diagnosis in the US, but a US researcher working with the Fukuoka team developed new criteria for the condition in 2010, the Journal notes. Experts are using that criteria to help develop a better understanding of the situation in a country where, the Times noted, mental illness remains highly stigmatized. (Another strange case: five siblings who walk on all fours.)