French scientists Charlotte Faurie and Michel Raymond of the University of Montpellier have found in a cross-cultural comparison of rates of left-handedness and violence that lefties occur at a higher rates in more violent societies.
Among the Jula (Dioula) people of Burkina Faso, the most peaceful tribe studied, where the murder rate is 1 in 100,000 annually, left-handers make up 3.4% of the population. But in the Yanomami tribe of Venezuela, where more than 5 in 1,000 meet a violent end each year, southpaws account for 22.6%. Faurie and Raymond report their findings in the Proceedings of the Royal Society.
Many sports are kinda like violence under civilized rules. Well, sports people are more often left-handed than the overall population.
And the ratio of left-handers to right-handers is higher in successful sportspeople than it is in the general population, suggesting there is definite advantage to favouring the left hand or foot in competitive games, such as tennis.
There are downsides to left-handedness.
Statistical evidence links several auto-immune diseases, such as inflammatory bowel syndrome and ulcerative colitis, with left-handedness.
Given the downsides there would have to be strong selective pressures to cause left-handedness to happen anyway. So the need to discover a selective pressure for left-handedness already existed before this latest report was published.
As any schoolboy could tell you, winning fights enhances your status. If, in prehistory, this translated into increased 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.
Suppose that, as this study suggests, the adaptive value of the ability to fight and win fights differed between societies so strongly that it selected for different frequencies of genetic variations that influence handedness. The existence of differences in selective pressures for competence in human-human violence is especially important because such differences in selective pressure would not have acted only on genes that influence the tendency to be left-handed. Such a difference in selective pressure for violence would also have produced differences in the frequencies of many other genetic variations that influence other aspects of the capacity to commit violent acts (e.g. tendency to impulsivness, ease of angering, muscle strength, and other cognitive and physical qualities).
By Randall Parker at 2004 December 10 03:05 AM Trends, Human Evolution | TrackBackVery interesting data. However, I'm not sure I buy the "surprise" interpretation: southpaws get their hand-to-hand combat advantage by surprising their opponents, who are used to encountering righties.
Even if this where true at first, when the percentage of lefties in the population was small, I would expect that in the long run, selective pressures would bring the final percentage of lefties to 50%. Why? Because then, on average, fighters would encounter just as many lefties as righties, and the surprise advantage disappears.
Perhaps the societies looked at had not yet gone through enough generations to reach my proposed equilibrium state. Or perhaps some of the disadvantages balance the "surprise" advantage and the equilibrium state is lower than 50%.
It would be interesting to look at a sport such as boxing over the last 100 years and see if the percentage of southpaws is asymptotically approaching 50%.
I forgot to write: Or perhaps my proposed equilibrium state is falsely reasoned.
No, the reason the equilibrium point is not at 50% is that lefthandedness exacts other costs. The mutations that are creating the lefties are creating lefties out of a basically righthanded design. So the overall design works better when the mutations that create lefties are not there.
An excellent example of wild speculation in the name of science ... the logic here is sad ... we need to speak of this trait over thousands of years to make an evolutionary argument ... the use of modern stats as an basis for initiating the conversation is exactly where the problem begins ... fighting and accidents in modern life are irrelevant as a selection basis ... I would contend that even the accident rate may be insufficient for selective pressures ... this is one of the most significant traits driving human evolution? ... I wouldn't bet on it ... the use of stats that to suggest higher incidences of disease for left handers is also not worth making ... what diseases are more common to right handers? This appears to be a legal style argument rather than a peice of scientific reasoning
P,
You say:
fighting and accidents in modern life are irrelevant as a selection basis
Are you saying that fighting isn't a selection pressure over hundreds or thousands of years?
Or is your problem with the assumption that for a long time the Yanomamo have been more violent than the average group? That strikes me as a plausible assumption. Even the wars of the 20th century did not produce death rates from violence that are as high as what has been measured among the most pre-industrial violent tribes.
Accidents surely are selective pressures. People who have more cautious personalities are less likely to speed or go skydiving or climbing mountains. Whatever the selective pressures were that selected for thrill-seeking in a portion of the population do not look like they are present any longer in modern societies. So I expect the higher death rates of thrill seekers will now be a larger selective pressure against them.
Pretty danged sinister.
Degustibus,
I had to shine polarized light on what you wrote to see the levulo twist in it.
Is left handedness hereditable? It seems to be so rare and random... My son is lefty, but we have very little history of left handedness in the family.
Do twins always have the same handedness?
-d
I found this about "mirror" twins on a twins website. It is interesting that they claim twins are either mirrors or right handed, but it may just be that double left handed twins are extremely rare.
However, I have heard the theory that lefties started out as twins and the right handed twin did not make it. Very interesting. But one would think that the lefty would then go on to pass on his left handed genes without the need to have twins. Maybe there are two mechanisms at work.
-b
http://www.twinsworld.com/ubbcgi/ultimatebb.cgi?ubb=get_topic&f=9&t=000040
--
Hello all you Mirrors !
We are another subset of twins more unique and different than the non mirrors. I/We (Barry and Gary Murphy) never met mirrors until we went to the 2004 Twinsburg festival last week. What a great surprise!
We began asking the TWIN birth order of all mirrors we met (10 sets - 4 female, 6 male) and found out that every left handed twin of the mirrors was born first, including me(Gary), I have been unable to find statistics of the number of left handed twins born first vs. right.
There are many hypothesis about the cause of mirrors including a statement that blames lack of oxygen and brain damage to the second birth as the cause for left handedness. We could not find a left handed mirror twin that was born second, so that theory goes out the window for out small research group.
Of the 50 + twins we questioned during the festival about handedness, we found no set of twins that were both left handed. All twins we questioned were both right handed or mirror.
Hope to see more data on mirrors soon.
Gary and Barry Murphy
jmgordon:
The advantage enjoyed by southpaws isn't really a matter of surprise, but rather of greater experience. Lefties are accustomed to fighting right-handers; right-handers, however, get much less experience fighting left-handers. This discrepancy was likely even more pronounced in premodern times, when there was less opportunity for practice outside of actual combat.
This, in fact, may be another limiting factor: well before the proportion of lefties reaches 50%, their comparative advantage in combat may end up reduced to insignificance.
Incidentally, I speak from some experience here, having spent many of my younger years training in various combat arts, armed and unarmed. early on, some of my closest sparring partners were left-handed, so I got to see not only what it was like to face a lefty, but how other righties with less exposure handled it themselves.
David,
I too spent a good deal of time in the ring (boxing, tae kwon do, kickboxing). In fact I still do a moderate amount of training. Unfortunately, I never realy paid attention to whether the southpaws gave me a harder time or not. Thus I can't add my data point. :(
One thing we can be sure of: most of those anthropologists probably didn't spend much time in the ring themselves!
My sense regarding armed vs unarmed combat is that left-handedness makes a bigger difference when weapons are involved. Usually the weapon goes in the strong hand, the other side being relegated primarily to defence. There are exceptions, of course, but that's the general pattern.