In Japan, there is a commonly held if bizarre belief that your blood type can determine your personality. Type As are, apparently, anxiety-ridden perfectionists who crave order and are prone to extreme fretting, whereas Bs are passionate creatives with a selfish tendency. Os, on the other hand, are confident and friendly but can be unpredictable and aggressive, while ABs are just eccentric.
The theory is so widespread, there are blood type horoscopes, blood type dating agencies, and even blood type chewing gum. Some employers may go so far as to ask prospective employees what blood type they are.
Of course, this is bullshit. Much like the alignment of stars on your birthday, your particular blood type has very little to do with your character traits. There is, however, at least one aspect of your personality that may be determined by the structure of your blood and that is your propensity to violence. A study recently published in the Proceedings of the National Academy of Sciences analyzed the behavior of mice injected with the blood of violent criminals – and found that the new blood really did make the animals more aggressive.
It could all come down to a particular protein involved in the production of cortisol, which regulates the body's response to stress.
After noticing differences in the blood composition between violent aggressors and healthy controls (i.e. those with no criminal records and a steady job), researchers from Akershus University Hospital in Norway siphoned antibodies from 16 prisoners who, with the exception of one, were in the middle of serving long-term sentences in high-security prisons having committed extremely violent crimes, including rape and murder. These antibodies were then injected into mice.
The transfusion transformed the otherwise healthy mice into aggressive antagonists, who were far quicker to engage in violence against other mice – in fact, they were four times quicker to jump on "intruders".
“The resident would attack the mouse very fast,” Sergueï Fetissov, the project's lead researcher, told The Times. “The antibodies may predispose people to aggressive behavior.”
This is a small-scale study so more research is needed to confirm the researchers' findings and work out why exactly someone's blood might make them more aggressive. As the researchers also point out, there may, in fact, be many people with the same variation in blood structure who do not display such aggressive behavior.
Yet, it does help explain some of the "mechanisms" behind criminality and may eventually lead to a "cure" to fix violent behavior, even if that does sound a little Clockwork Orange.