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On the comparative advantage of left-handed violence
Left-handedness is bad for you, apes are all ambidextrous, hominids are somewhat right-handed1 and today only 8%-10% of the population uses the left. This means that there has been a long ongoing selection for the use of the right, we can see in cohort data that this process of selection has continued until recent times. Even today there are genetic correlations between genes that reduce fitness, that is variants that cause psychiatric illness, and left-handedness.
So if LH is bad for your fitness why is it still around? (
also if we all came from ambidextrous apes how come there are still ambidextrous apes around?) After all, there have been millions of years for selection to operate and raise RH to fixation. The answer must be that LH holds some advantages that have kept the trait ongoing in the population in non-trivial frequencies.
One theory for LH advantage is “Frequency-Dependent Maintenance of Left-Handedness in Humans” by Michel Raymond, the idea is that in groups that face high rates of physical combat being left-handed may grant an advantage because a left-swinger is used to fighting right-swingers, who are more common in the population, while they are not used to fighting him. Under those conditions, the rate of left-handedness will rise in the population until it will become common enough to eliminate the advantage of having an unusual fighting style.
Several things are going for this theory, The fiercer sex is more LH than the fairer sex. In the largest study on this subject, a national geographic survey of 1.4 million Americans, LH was 25% more common in men than in women and that difference remained constant across birth cohorts2.
There is a large over-representation of LH in sports in which the athletes are facing an opponent, while the same is not true for sports in which athletes are bowling alone.
This data from Raymond’s original article and further studies have confirmed it, such as this study of senior athletes in the top 200 rankings:
Looking at fighters in particular, a study of 13,800 professional boxers and mixed martial artists showed not only greater LH among fighters but also a higher probability of winning. (although over-representation in a sport may constitute better evidence for greater ability than higher probability for victory, this is because one can imagine a situation in which enough left-handers enter a sport until LH is no longer an advantage)
People’s natural intuitions are in line with this, among people who thought handedness mattered for the chance of winning a fight, 79.8% thought left-handers were more likely to win3.
If the present intution of Lefties’ combat ability extended to the past, It may explain some peculiar historical references such as when Judges 20:16 brag about having 700 left-handed warriors who could ‘sling a stone at a hair and not miss’ and when Chronicles 12:2 reference bowmen who were ambidextrous.
It could also offer some explanation about anti-left-handed bias, the common custom of a handshake, likely invented to present peaceful intentions by showing the dominant hand is not carrying a weapon, fails with lefties and the checking of weapons on the right side of the body has not saved Eglon, king of Moab from being stabbed by Ehud the israelite4 (If you ask me the king’s guards were slacking off).
Left-handedness among the hunters-gatherers
If the frequency of LH is maintained due to a combat advantage, do we see elevated levels of LH among some highly violent societies? there is some non-conclusive for this, in a small sample of traditional societies higher rates of LH are seen in tribal groups with higher rates of homicide (Handedness here is not the writing hand but the knife/machete hand or tool-using hand). Interestingly, the Yanomamo, A group in which Napoleon Chagnon showed Men who committed homicide have higher rates of reproductive success5, have the highest rates of LH.
We can look at some further tentative data, possibly there is a way to compare LH in agricultural and pre-agricultural societies. A reconstruction of LH in historical societies6 was created by looking at statues and paintings in which a tool or a weapon is in use:
This could be compared to reconstructions of handedness in pre-agricultural societies, done by comparing the frequency of left-handed hand stencils to right-handed hand stencils in cave painting7:
The assumption here is that the left-handed painted right-hand stencils and vice versa, but there are some problems of interpretation here, “Faurie C, Raymond M. (2003)” got a group of people to draw hand stencils with a blow-pipe and got frequencies consistent with those in the Gravettian caves, this was because the left-handed people draw right-handed stencils while the right-handed did not do the reverse as much. While in some respect such an experiment is very compelling on the other
hand side it’s hard to see these results as anything but the consequence of chance, it is also not clear how such an experiment would replicate in various caves with differing proportions of left and right prints. N. Uomini & J. Steele8 also mention criticism of the experiment, pointing out that the placement and orientation of various prints in the cave would make it impossible to paint them using a blow-pipe which would make it harder for people to make a print of their dominant hand.
So the Left-handed advantage in violence seems fairly well supported by the athletic studies while its effect on the proportion of LH across various populations has something going for it, but more stabbing are needed.
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McManus, I C. “The history and geography of human handedness.” (2009). - also see this meta-analysis “ of k = 144 studies, totaling N = 1,787,629 participants, the results of which demonstrate that the sex difference is both significant and robust. The overall best estimate for the male-to-female odds ratio was 1.23”
Faurie C, Laurens V, Alvergne A, Goldberg M, Zins M, Raymond M. Left-handedness and male-male competition: insights from fighting and hormonal data. Evol Psychol. 2011.
Reproductive success among the Yanomamo:
This reconstruction is not granular enough to see the interesting pattern of the fall and rise of 19th/20th-century LH. 19th-century reconstructions of LH are quite ingenious, using methods like counting the waving hand of people in old photographs.
France & Spain: Delluc & Delluc (1993,34-5); Australia, Americas, Europe: Kirchner's study (1959,110); Indonesia, Kalimantan caves: Plsgnrd et al.2003; South America, Patagonia: Gradin (1994, 154); USA, Montanna: Greer & Greer (1999, 60); Australia, levy ranges: Gunn (1998).