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Why are some people left-handed?

Have you ever wondered why left-handed people are so rare? Everyone knows that right-handed people are more frequent than left-handed people. You may have heard different theories about how the handedness is determined, but you probably never heard of a theory explaining why left-handed people are so rare. Daniel M. Abram found a way to explain the small number of left-handed people with a mathematical model.

Only one out of ten people is left-handed. The fascinating thing is that this ratio has remained steady for the last 500’000 years. Until today, it is not exactly clear what determines the handedness. One thing we know, however, is that the handedness is nothing you can choose, it is given. Many different theories try to explain this. The observation that left-handed parents tend to have left-handed children more often than right-handed parents, can be explained either by the influence of genes or the environment. Studies with identical twins show that both genes and the environment influence handedness, since identical twins do not have the same dominant hand more often than other siblings do.

This implies that there must be a reason in evolution responsible for the small number of left-handed people. Daniel M. Abrams proposed a mathematical model, which suggests that the ratio of competitive and cooperative pressure is responsible for the small number of left-handed people. 

The advantages of left-handed people are most obvious when facing an opponent in combat or competitive sports. Because there are usually very few left-handed people, most athletes are used to train with right-handed people. When right-handed and left-handed people meet, the left-handed person will be better prepared than a right-handed opponent. Daniel M. Abrams showed that 50% of professional baseball players are left-handed. The imbalance, in the beginning, leads to an advantage for left-handed players. This is called the fighting hypothesis and is an example of negative frequency-dependent selection. The rarer a trait, the more valuable it is. But according to the rules of evolution, a group that has an advantage should grow until the advantage disappears. If all humans did was fight, natural selection would lead to more left-handed people. The number of left-handed people would grow until there would be so many of them that it would not be rare anymore and therefore left-handedness would not be an advantage anymore. Thus, in a purely competitive world, the ratio between left and right-handed people would be 50/50. 

However, human evolution is not only driven by competition but also by cooperation. Cooperative pressure pushes the handedness in the other direction. In golf, where performance does not depend on the opponent, only 4 percent of the top players are left-handed. The reason for this is a phenomenon called “tool sharing”. Many products and tools are made for right-handed people, as they also make up the majority of our society. Left-handed players are worse at using these tools. For this reason, left-handed people would be less successful in a purely cooperative world until they would no longer exist. 

To summarize, according to Abram’s mathematical theory, the stable number of left-handed people is seen as an equilibrium created by competitive and cooperative effects. 

  • Abrams, D., (2015, February). Daniel Abrams : Why are some people left-handed ? Retrieved from
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  • Wiedmer, J., (2020). Handmade.

Author : Jessica Wiedmer