Left-Handers Are Better Fighters than Right-Handers

New large-scale study sheds light on an old question in evolutionary psychology

Posted Dec 22, 2019

About 10% of people are left-handers, while the remaining 90% are right-handers. These numbers are relatively stable over the last 5000 years (Coren & Porac, 1977). While we know that left-handedness is determined by a combination of environmental factors and genes (see my blog posts here and here), this does not explain the evolutionary reason for the existence of left-handedness.

Why are most people right-handers, but there is a low but stable percentage of about 10% of left-handers?

One widely cited idea to explain this pattern is the so-called fighting hypothesis (Raymond et al., 1996). This hypothesis argues that left-handedness represents an evolutionary advantage, but only as long as it is much rarer than right-handedness. The basic idea is that right-handers do not have much experience in fighting left-handers, because they are so rare. In contrast, left-handers have much more experience in fighting right-handers as they are much more common than left-handers. Thus, left-handers have an advantage in fighting over right-handers, but only as long as left-handedness is rare. As being a left-hander in a world in which many things that are designed for right-handers could also have some negative evolutionary consequences, e.g. higher rate of injuries when using tools designed for right-handers, there would also be an evolutionary force against left-handedness. Combined, these two factors explain the 10% rate of left-handers.

How can the fighting hypothesis for left-handedness be tested empirically?  

So far, evidence for the fighting study mainly came from studies that investigated whether left-handers are overrepresented among professional fighters. For example, it has been shown that left-handers are over-represented in the Ultimate Fighting Championship (Pollet et al., 2013) or in wrestling (Ziyagil et al., 2010). Unfortunately, these studies had one drawback: Just because there were more left-handed fighters in many sports does not necessarily mean that they were also more successful at fighting than right-handers. To prove that the fighting hypothesis is correct, researchers would need to assess whether or not left-handers actually win more fights than right-handers, irrespective of their numbers in a specific sport. In a new study published in the scientific journal Scientific Reports, Thomas Richardson and R. Tucker Gilman from the School of Earth and Environmental Sciences at the University of Manchester (UK) now did exactly that (Richardson & Gilman, 2019). Importantly, Richardson and Gilman (2019) used a much larger sample than previous study to assess their research question. Overall, their data pool consisted of more than 13,800 professional fighters (10,445 male boxers, 1314 female boxers and 2100 mixed martial-arts fighters).

What were the results?

The authors found strong evidence supporting the fighting hypothesis. First, they could replicate the finding that left-handers are overrepresented in professional combat sports. They found that in male boxers 17% were left-handers, in female boxers 12.5% were left-handers and in mixed martial-arts fighters 18.7% were left-handers. Compared to the roughly 10% of left-handers in the general population, these numbers were significantly higher when tested statistically.

Importantly, the authors also found evidence that left-handed fighters actually do have greater fighting success. When looking at the percentage of fights won, Richardson & Gilman (2019), also found that in all three investigated groups, left-handers would have an average chance higher than 50% to win a fight. For male boxers, the win percentage was 52.4%, for female boxers it was 54.5% and for mixed martial-arts fighters 53.5%. When compared statistically, all of these numbers were significant, showing that left-handed fighters are actually more successful than right-handed fighters.

What do we learn from this?

In conclusion, these results show that left-handers are not only overrepresented in professional combat sports, but also actually do win more fights. This supports that fighting hypothesis and suggests that superiority in fighting might be one of the evolutionary reasons for the existence of left-handedness (but there might of course also be others).

References

Coren S, Porac C. (1977). Fifty centuries of right-handedness: the historical record. Science, 198, 631-632.

Pollet TV, Stulp G, Groothuis TGG (2013). Born to win? Testing the fighting hypothesis in realistic fights: left-handedness in the Ultimate Fighting Championship. Animal Behaviour, 86, 839-84

Raymond M, Pontier D, Dufour AB, Møller AP. (1996). Frequency-dependent maintenance of left handedness in humans. Proc Biol Sci, 263, 1627-1633.

Richardson T, Gilman RT. (2019). Left-handedness is associated with greater fighting success in humans. Sci Rep, 9, 15402.

Ziyagil MA, Gursoy R, Dane S, Yuksel R. (2010). Left-handed wrestlers are more successful. Percept Mot Skills, 111, 65-70.