December 15, 2004

Left-handers and homicide rate

This exciting new research discovered that the prevalence of left-handedness is correlated with the homicide rate in different human populations. The explanation proposed by the authors is that left-handers have an advantage during a fight, because they can surprise their opponents. This is supported by research in competitive sports where two individuals "fight" each other. However, in the past, fights were often to the death, and hence the advantage of left-handers would be selected, because they could fool their opponents more easily.

But, this ability to "fool" one's opponent is strongest when left-handedness is rare, i.e., when the right-hander doesn't expect it. As the frequency of left-handedness increases, the advantage of left-handers will gradually decrease. The equilibrium point will be different in different human societies depending on how "warlike" they are. In completely peaceful societies, no fights occur and hence left-handers have no advantage. In a very warlike society, left-handers get to kill their unfortunate right-handed opponents more frequently; but the frequency of left-handedness cannot increase unchecked, because its effectiveness diminishes as more and more people are using the "trick" of giving left-handed strikes.

The authors looked at eight traditional societies and discovered that left-handedness and the homicide rate, a measure of violence in a society are correlated. This gives strong support to this theory for the evolution of left-handedness.

Proceedings: Biological Sciences (FirstCite)

Handedness, homicide and negative frequency-dependent selection

Charlotte Faurie and Michel Raymond


Humans exhibit hand preference for most manual activities in which they are specialized. Right- and left-handers have coexisted at least since the Upper Palaeolithic, and left-handers are in the minority in all human populations. The persistence of the polymorphism of handedness is a puzzle because this trait is substantially heritable and several fitness costs are associated with left-handedness. Some countervailing benefit is required to maintain the polymorphism. Left-handers may have a frequency-dependent advantage in fights--the advantage being greater when their frequency is lower. Sports data from Western societies are consistent with this prediction. Here, we show that the frequency of left-handers is strongly and positively correlated with the rate of homicides across traditional societies. It ranges from 3% in the most pacifistic societies, to 27% in the most violent and warlike. This finding is consistent with a frequency-dependent selection mechanism maintaining left-handedness in these societies.


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