I emailed an old friend of mine in Japan. He works to coordinate Western tourists with Japanese guides, who work only for cost of travel and the chance to practice English. I asked him if he could let me know the name of the type of business I referred to earlier, or anything else which might help. I'll let you know...
It's a big business in Japan, investigating the other family... sadly, the business is based on racial discrimination more than anything else (as I've had it explained a few times; maybe things have changed in 15 years). There is an ethnic group called Burakumin, the Japanese equivalent of the Native American Indians, which people quietly do not wish to intermarry with. (I think some people want to make sure they are not marrying someone who has roots in another country, either.) Employers, I think, also do some quiet investigating sometimes. To talk about Burakumin at all is almost like talking about something highly embarassing; the folks who I asked about this were anxious to keep their voices low, as if they were sharing a dark secret. For this and other reasons, the Japanese are meticulous record-keepers.
bebu
Edited to add: I am sorry if I'm implying that this is the primary reason for their great record-keeping. I think they are good record-keepers to start with, just because they love order so much.