Or perhaps you prefer the short version: lefties are killers.

That's the gist of a provocative new study by Charlotte Faurie and Michel Raymond of the University of Montpelier [France], just published in the Proceedings of the Royal Society.

The authors note that there are significant evolutionary disadvantages to being left-handed.

Among them: lefties are, on average, smaller and lighter than righties; being left-handed is accompanied by significantly more immune-system abnormalities and deficiencies.

Therefore, "some countervailing benefit is required to maintain the polymorphism."

Translation: there must be some advantage to being left-handed that outweighs the known negatives.

And there is.

It turns out that in the most violent traditional societies, such as the Yanomamo of South America, the murder rate is 4 per 1,000 inhabitants.

22.6% of the Yanomamo are left-handed.

In contrast, Dioula-speaking people of Burkina Faso in West Africa have only 0.013 murders per 1,000 inhabitants.

Their left-handers number only 3.4% of the populace.

Faurie and Raymond note that left-handed fighters have a strategic advantage in close combat, simply because most right-handed people have little experience in fighting lefties.

If, they speculate, in pre-history this translated into increased survival and reproductive success, it might have been enough to maintain a certain proportion of left-handers in the population, by balancing the costs of being left-handed with the advantages gained in fighting.

While there is no suggestion that left-handed people are more violent than the right-handed, it appears they are more successfully violent.

So the next time you're faced with someone wielding a weapon, and it's in the other person's left hand, you might want to take this into account as you calculate the answer to your own personal fight-or-flight equation.

[via the Economist]