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Left-Handedness: Understanding Why People Are Left-Handed

If you know an older left-handed person, chances are they had to learn to write or eat with their right hand. And in many parts of the world, it’s still common practice to force children to use their “proper” hands. Even the word for right also means correct or good, not just in English, but in many other languages, too. But if being left-handed is so wrong, then why does it happen in the first place?

The Origins of Left-Handedness

Today, about 1/10 of the world’s population are left-handed. Archeological evidence shows that it’s been that way for as long as 500,000 years, with about 10% of human remains showing the associated differences in arm length and bone density, and some ancient tools and artifacts showing evidence of left-hand use. And despite what many may think, handedness is not a choice. It can be predicted even before birth based on the fetus’s position in the womb.

The Genetic Puzzle of Handedness

Identical twins, who have the same genes, can have different dominant hands. In fact, this happens as often as it does with any other sibling pair. But the chances of being right or left-handed are determined by the handedness of your parents in surprisingly consistent ratios. Handedness seems to be determined by a roll of the dice, but the odds are set by your genes.

Evolutionary Perspectives on Left-Handedness

The benefits of being left-handed are clearest in activities involving an opponent, like combat or competitive sports. For example, about 50% of top hitters in baseball have been left-handed. Why? Think of it as a surprise advantage. Because lefties are a minority, to begin with, both right-handed and left-handed competitors will spend most of their time encountering and practicing against righties. So when the two face each other, the left-hander will be better prepared against this right-handed opponent, while the righty will be thrown off.

Competitive and Cooperative Pressures

However, according to the principles of evolution, groups that have a relative advantage tend to grow until that advantage disappears. If people were only fighting and competing throughout human evolution, natural selection would lead to more lefties being the ones that made it until there were so many of them, that it was no longer a rare asset. So in a purely competitive world, 50% of the population would be left-handed. But human evolution has been shaped by cooperation, as well as competition. Cooperative pressure pushes handedness distribution in the opposite direction.

Equilibrium of Handedness

In golf, where performance doesn’t depend on the opponent, only 4% of top players are left-handed, an example of the wider phenomenon of tool sharing. Just as young potential golfers can more easily find a set of right-handed clubs, many of the important instruments that have shaped society were designed for the right-handed majority. Because lefties are worse at using these tools and suffer from higher accident rates, they would be less successful in a purely cooperative world, eventually disappearing from the population.

The Fascination with Left-Handedness

By correctly predicting the distribution of left-handed people in the general population, as well as matching data from various sports, the model indicates that the persistence of lefties as a small but stable minority reflects an equilibrium that comes from competitive and cooperative effects playing out simultaneously over time. And the most intriguing thing is what the numbers can tell us about various populations. From the skewed distribution of pawedness in cooperative animals to the slightly larger percentage of lefties in competitive hunter-gatherer societies, we may even find that the answers to some puzzles of early human evolution are already in our hands.