Kenji Kawamoto, a Tokyo office worker who organizes various social gatherings in Tokyo as a hobby, has been busy organizing one particular holiday party this year. You could say it's in his blood.
"The success of this party is all I have been thinking of nowadays," the 42-year-old said excitedly, like a little child waiting for Santa's arrival. "We had a pretty large crowd last year too, but this year's should be even bigger. We have more than 100 people signed up for this event."
Kawamoto created a social group 13 years ago when he was new to the city, and he's been coordinating parties ever since.
"When I first came to Tokyo for my job, I had no friends and I wanted to meet people," Kawamoto said. "So I created a social group calling for people from the Kyushu region [of southern Japan] where I am from. The group kept getting bigger, and we now have other social groups with names like People Born in the '70s or People Living in North Kanto region [Tokyo vicinity]."
Which group is drawing the biggest crowd this year? "Oh, that would be the blood type group. This group kept growing this year and we saw many new members."
Japanese are often asked their blood type in various settings. Friends often try to guess each other's blood type or one may be asked to mention his or her blood type on a job application. In singles' bars, it can be a common question -- the Japanese equivalent of "What's your sign?"
Many Japanese people believe blood type is an indicator of everything from personality to marriage compatibility.
Japanese morning television shows and magazines often include horoscopes based on blood type and several books on blood type analysis came out during the last year, including one called "An Instruction Manual for Those With Type B Blood."
The book, written by an author who goes by the pen name of Jamais Jamais, became one of the nation's top sellers. The manual led to sequels for other blood types -- A, O and AB -- selling a total of 5 million copies.
"Blood type is a good tool to understand different human behaviors," said Chieko Ichikawa, the head of Human Science ABO Center in Tokyo. The center was founded by journalist Masahiko Nomi, who released a series of publications on blood type in the 1970s with a concept called blood type humanics -- a new way of studying the human mind.
"Most Japanese have some knowledge of each blood type and its trait so blood type often becomes a good conversation piece," Ichikawa said.
Most scientists don't see a basis for the nationwide fascination with blood types, at least for now.
"There does not seem to be any validity between blood type and personality or traits analysis," said Toshiki Nishizawa, a clinical psychologist in Tokyo. "Having said that, dating back to 400 B.C., the days of the Greek physician Hippocrates... people tried to study if or how fluids in our body such as blood can affect our personality."
Kawamoto's group called Association for Blood Type A.B.O.AB meets almost every month, with roughly 40 attendees at each gathering.
"This is just one way to meet new people and make friends," Kawamoto said.
And it's been a fruitful experience for some.
"We have 37 couples that met at those gatherings and got married," Kawamoto said. "I met my wife through group activities and we are the 22nd couple. We are both Type O and we have been getting along pretty well."