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Dr. Daniel Abrams, Northwestern University – Cooperation, Competition, and Lefties

In today’s Academic Minute, Dr. Daniel Abrams of Northwestern University explains why the level of left handedness is a constant across most societies.

Dr. Daniel Abrams is an assistant professor in the Department of Engineering Sciences and Applied Mathematics at Northwestern University where his research interests include nonlinear dynamics, mathematical geoscience, physics of social systems, and pattern formation. He holds a Ph.D. from Cornell University.

About Dr. Abrams

Dr. Daniel Abrams – Cooperation, Competition, and Lefties

Why is it that in every modern society about 1 person in 10 is left-handed?  Archaeological evidence shows it's been that way for at least 5000 years, and probably much longer.  So lefties are here for a reason: evolution has produced them in just this ratio and maintained it over the course of millennia.

Recently, a colleague and I came up with a mathematical model that can explain why this happened: left-handedness comes from a balance between competitive and cooperative effects during human evolution.  Let me explain.

The effect of physical competition is easy to see among top hitters in baseball, where historically about 50% have been left-handed.  Why? Think of it as a "surprise advantage:" when there are few lefties, they practice a lot against righties, but righties practice little against them.  This gives them an advantage, and—going back to human evolution now—groups that have a relative advantage tend to grow until that advantage disappears. So 50% of the population would be left-handed in a purely competitive world.

A different effect, due to cooperation, can be seen in professional golf, where only about 4% of top players are left-handed.  This is a result of "tool-sharing:" young potential golfers can more easily borrow a set of right-handed clubs than the comparatively rare left-handed version, and that leads to a small disadvantage for lefties.  In terms of human evolution, right-handed tools just don't work as well for lefties, and in a purely cooperative world, that disadvantage would lead to their eventual disappearance.

When both competitive and cooperative effects play out simultaneously, our model predicts that society should reach an equilibrium where lefties persist, but are rare: today's world.

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