Are you A, B, O or AB? It is a widespread belief in Japan that character is linked to blood type. What's behind this conventional wisdom?

Blood is one thing that unites the entire human race, but most of us don't think about our blood group much, unless we need a transfusion. In Japan, however, blood type has big implications for life, work and love.

Here, a person's blood type is popularly believed to determine temperament and personality. "What's your blood type?" is often a key question in everything from matchmaking to job applications.

According to popular belief in Japan, type As are sensitive perfectionists and good team players, but over-anxious. Type Os are curious and generous but stubborn. ABs are arty but mysterious and unpredictable, and type Bs are cheerful but eccentric, individualistic and selfish.

About 40% of the Japanese population is type A and 30% are type O, whilst only 20% are type B, with AB accounting for the remaining 10%.

What's your blood type? The main blood group system is ABO, with four blood types: A, B, O, AB

Rhesus system , for which you can be positive or negative , is the second most important with regard to blood transfusions

, for which you can be , is the second most important with regard to blood transfusions In total there are 32 recognised blood group systems , which all have either positive or negative indicators

, which all have either positive or negative indicators The discovery of the latest two blood types - Langereis and Junior - were announced by researchers from Vermont earlier this year

Four books describing the different blood groups characteristics became a huge publishing sensation, selling more than five million copies.

Morning television shows, newspapers and magazines often publish blood type horoscopes and discuss relationship compatibility. Many dating agencies cater to blood types, and popular anime (animations), manga (comics) and video games often mention a character's blood type.

A whole industry of customised products has also sprung up, with soft drinks, chewing gum, bath salts and even condoms catering for different blood groups on sale.

Blood types, however, are simply determined by proteins in the blood. Although scientists regularly try to debunk these beliefs, they remain popular in Japan. One reason often given is that in a relatively uniform and homogenous society, it provides a simple framework to divide people up into easily recognisable groups.

"Being the same is considered a good thing here in Japanese society," says translator Chie Kobayashi. "But we enjoy finding little differences that distinguish people. On the other hand, it can also lead to bad things being said about the minority B and AB types."

A minister quits In July 2011, Minister for Reconstruction Ryu Matsumoto resigned after being criticised for making insensitive remarks. He blamed his blood type. "I would like to offer my apologies for offending the people in the disaster-hit areas. I thought I was emotionally close to the disaster victims, but I lacked sufficient words and my comments were too harsh. "My blood's type B, which means I can be irritable and impetuous, and my intentions don't always come across. "My wife called me earlier to point that out. I think I need to reflect about that." Japan minister quits after a week

It was only in 1901 that the ABO blood group system was discovered by the Austrian scientist Karl Landsteiner. His Nobel prize-winning work made it possible to identify the different blood groups, paving the way for transfusions to be carried out safely.

Theorists of eugenics later hijacked his research during the inter-war years, with the Nazis using his work to further their ideas of racial supremacy.

It was also adopted by Japan's militarist government in the 1930s to train better soldiers, and during World War II, the Imperial Army is reported to have formed battle groups according to blood type.

The study of blood types in Japan gained mass appeal with the publication of a book in the 1970s by Masahiko Nomi, who had no medical background. More recently, his son Toshitaka went on to promote it further through a series of popular books - he also runs the Institute of Blood Type Humanics. He says his aim is not to judge or stereotype people, but simply to make the best of someone's talents and improve human relationships.

Between them, father and son have published dozens of books on the subject, not just the handful of bestsellers.

These beliefs have been used in unusual ways.

Societies dominated by B types are more prone to polytheism - like Buddhism and Hinduism - with lots of gods Professor Maekawa

The women's softball team that won gold for Japan at the Beijing Olympics is reported to have used blood type theories to customise training for each player. Some kindergartens have even adopted methods of teaching along blood group lines, and even major companies reportedly make decisions about assignments based on employees' blood types.

In 1990 the Asahi Daily newspaper reported that Mitsubishi Electronics had announced the creation of a team composed entirely of AB workers, thanks to "their ability to make plans".

These beliefs even affect politics. One former prime minister considered it important enough to reveal in his official profile that he's a type A, whilst his opposition rival was type B. Last year a minister, Ryu Matsumoto, was forced to resign after only a week in office, when a bad-tempered encounter with local officials was televised. In his resignation speech he blamed his failings on the fact that he was blood type B.

Not everyone sees the blood type craze as simply harmless fun.

It sometimes manifests itself as prejudice and discrimination, and it seems this is so common, the Japanese now have a term for it - bura-hara, meaning blood-type harassment. There are reports of discrimination against type B and AB groups leading to children being bullied, the ending of happy relationships, and loss of job opportunities.

Despite repeated warnings, many employers continue to ask blood types at job interviews, says Terumitsu Maekawa, professor of comparative religion at Tokyo's Asia University and author of several books about blood groups. He's critical about sweeping popular beliefs about blood types.

"We can point out some general tendencies as a group, but you can't say this person is good or bad because of their blood type."

Image caption Professor Maekawa has written several books about blood groups

His own research, he says, is based more on empirical research rather than popular superstition. In his books he explores the theory that predominant blood types may determine religious beliefs and societal norms.

In the Western world, O and A types make up almost 85% of people, but in India and Asia, B types predominate. Japan, he says, is unusual in Asia in that it has more variety of blood types.

They think I am weird and strange - lots of people tell me they don't understand what I am thinking about Masako, who has the rarer AB type blood

"A type societies tend to be characterised by monotheism such as Christianity and Judaism, with one fundamental analysis of human beings and a strong sense of societal norms. But societies dominated by B types are more prone to polytheism - like Buddhism and Hinduism - with lots of gods, and they think people are all different."

Professor Maekawa, himself type B, says in Japan his blood group is often criticised for being too individualistic and selfish.

"It isn't very nice. But it doesn't annoy me or hurt me, because it has no scientific basis at all."

In a smart state-of-the-art clinic busy with lots of people donating blood, director Akishko Akano says he's not aware that the negative image of certain blood types has an impact on their work, or dissuades minority B and AB types from coming forward. A bigger problem in Japan's rapidly ageing society, he says, is persuading enough young people to volunteer as blood donors.

Image caption Masako, who has type AB, has donated blood eight times

In the next room, I find Masako, lying on a bed strapped to a quietly purring machine as a nurse takes samples. This is the eighth time she's given blood. Her blood type is AB, which is rare as it accounts for only 10% of people in Japan.

"People sometimes don't like me," she tells me. "They think I am weird and strange. Lots of people tell me they don't understand what I am thinking about."

Although Masako laughs as she tells me this, it seems that in Japan, no amount of scientific debunking can kill the widely held notion that blood tells all.