June 6, 1980, was a Friday. The Social Insurance Agency quietly issued an untitled internal memo called a naikan regarding the eligibility of part-timers in Japan's shakai hoken health and pension program. Who could have known what chaos, confusion and frustration that single-page document would cause in the coming decades? Let's get our hands dirty and dig through the details.

All residents of Japan, regardless of nationality, must be enrolled in universal public health care and pension programs, in one of two parallel systems — one for employees, the other for the rest. The one for employees combines both health insurance (kenkō hoken) and pension (kōsei nenkin) into a set called shakai hoken (officially called Employees' Health & Pension Insurance in English). It's better than — and incorporates — the other schemes (kokumin kenkō hoken, aka National Health Insurance, and kokumin nenkin, the National Pension). By better, I mean it offers better benefits, as well as requiring employers to put up half the monthly premiums.

What are the better benefits? Shakai hoken includes two-thirds coverage of lost wages due to illness (shōbyō teate) beginning on the fourth day of sick leave and continuing for up to 18 months. Like its kokumin counterpart, shakai hoken covers 70 percent of medical costs. Both old-age and disability pension benefits for those in kōsei nenkin (the pension portion of shakai hoken) are far more generous than for those in kokumin nenkin. (Full disclosure: I receive the disability pension under the kōsei nenkin scheme.)