Japan would do better against "Hikikomori syndrome" by changing the studding environment culture of their school: bulling against the non-normative people (with closed eyes from the school), super competitive studies, almost mandatory second school, all to become boring salarymen bullied by the corporate higher ups. The society pressure for this life model is the problem, not some sort of genetics.

Also, most Hikikomori are men, because women may be able to escape all this crap by getting married. This is also a failure, because they are not expected to get a "real work" and to quit asap if married.

There is also the "herbirore" phenomenon, the cousins of the Hikikomoris, mainly people who want to avoid the family culture of Japan. (And let not forget about the suicides, but japan is not ranked first on this contrary to common beliefs)

Fortunately all this seems to change little by little, but searching for "biological roots" is just searching for excuses. Next research will be an American university searching for "biological root" in young people doing killing spree in schools ?