As far as Asian birth predictions go, most Westerners are familiar with the animal horoscope by year. (Primarily, it must be said, via the medium of Chinese restaurant placemats.) But few Westerners are aware that your blood type is taken much more seriously than your birth year animal.
There are four human blood types: A, B, O, and AB. Each blood type is said to have certain traits, in much the same way that your astrological horoscope sign is said to determine your personality. People with type A blood are thought to be creative and sensible, but also tense and conservative. People with type O are the outgoing social type, and so forth.
This pervasive belief in the power of blood types can be seen everywhere in contemporary Japanese society. Japanese morning news shows will often read off quick daily horoscopes for blood types. Japanese dating sites and matchmakers put a lot of stock in blood type compatibility charts. At one time, employers included a field for blood type on their job applications. Applicants were then sorted into departments by blood type.
As a measure of how pervasive this belief is, consider that Facebook Japan is the only Facebook regionalized site which includes a field on your profile for blood type!
For the most part, this belief in blood type horoscopes is harmless. However, it has historically led to discrimination based on blood type. People with type AB blood are said to combine the worst traits of type A and type B blood.
The Wikipedia article lists the worst traits for AB blood type as being "critical, indecisive, forgetful, and irresponsible." That hardly sounds like the kind of person you would want to hire or marry, right? And thus, people with type AB blood have often been discriminated against, without them ever having a chance to demonstrate their skills or actual personality traits.
In the late 1920s, two Japanese scientists published a wholly unscientific (and surprisingly racist) scientific paper about personality groups based on blood type. The research behind the paper tied into the Nazi regime, which spent a lot of time thinking about blood types and blood purity.
Japan lost interest in blood type from the 1930s until it was revived in the 1970s by a series of very popular books. And it has remained popular today, with a recent set of four blood type personality books occupying four slots on the "top 10 bestsellers" list for 2008.
If it would be odd for someone in the West not to know their horoscope sign, it is considered similarly odd in Japan for someone not to know their blood type. In fact I have been told that if a Westerner visiting Japan doesn't know their blood type, many Japanese people assume that they do know it, but don't want to admit to having an unflattering or unpopular type! (Protip: just say you're type A.)
Photo credit: Flickr/carpediem37