In Japan, "What's your type?" is much more than small talk; it can be a paramount question in everything from matchmaking to getting a job. By type, the Japanese mean blood type, and no amount of scientific debunking can kill a widely held notion that blood tells all.

In the year just ended, four of Japan's top 10 best-sellers were about how blood type determines personality, according to Japan's largest book distributor, Tohan Co. The books' publisher, Bungeisha, says the series u2014 one each for types B, O, A, and AB u2014 has combined sales of well over 5 million copies.