The blood type group and other social groups often meet for various activities such as barbecues in summer and cherry blossom viewing in spring.
"People sometimes use blood type analysis to see who may be a better match for them," Ichikawa said. "Such knowledge should be used only as guidance and not a definitive last word. But knowing one's blood type is like doing your homework. It may better prepare you when you meet a stranger or give some reasoning for the behavior of someone, which you may find hard to understand."
Takeshi Yoshida, another member of the group who married a fellow group member, said his wife, Ayako, surprised him occasionally when they first started to live together.
"I am Type A who is said to be a lot more detail-oriented than Type B, my wife," Yoshida said. "For example, when I hang laundry, say a handkerchief, I pull out the wrinkles and then fold it nicely before I hang it. This way I can just pick it up in the morning before I head off to work. My wife still hangs it as one big piece even though she realizes why I do what I do."
Yoshida understands every couple may go through similar experiences, and he said he sometimes finds himself referring to blood type analysis to understand differences between him and his wife.
"The widespread use of blood type analysis in Japan is not that surprising," said Ichikawa. "We have all four types spread out among the population -- 31 percent Type O, 38 percent Type A, 22 percent Type B and 9 percent Type AB -- so taking samples and analyzing them is much easier in Japan. In some parts of the world, like many countries in North America and Europe, a few blood types -- specifically Type O and A in this case -- make up a majority."
"Blood type analysis may give people a framework to help them understand society and people," the clinical psychologist Nishizawa said. "Having a frame of reference gives people peace of mind. This may reflect the state of our society -- uncertain and fluid. From job security to human relationships, many people do not know what to believe or where things are going ... and blood type analysis may just do that for them."
The Japanese have long studied and analyzed traits of human behaviors based on four blood types -- A, O, B and AB.
The study of blood types began in the early 1900s, soon after their existence was discovered.
Japanese scientist Takeji Furukawa published what is considered the nation's first book on blood typology in 1927. The nation's fascination with blood type and different behavioral tendencies grew with Nomi's extensive work in the field. Through his field work and observation, Nomi collected blood type data of more than 50,000 Japanese.
"Differences in the blood type composition can show unique traits or tendencies of people," Ichikawa said. "Our research has focused on Japan so far but it would be interesting to conduct research in those countries if we can collect enough data and samples to analyze."
Here are Ichikawa's descriptions of each blood type and some famous examples from the Human Science ABO Center:
Type O: Often seeks to make friends or form a group. Has great ambition for power and prosperity. Strives for leadership.