Why don’t left-handed persons make up half the population?

— Gayatri Chandrashekar

A: There’s a neural basis for handedness but it’s not due to any single part of the brain.

Hand preference reflects how the brain organises movement control. In most right-handed people, the left hemisphere has stronger control over fine hand movements and also more often dominates language. In left-handed people, these patterns are more mixed. Studies have found average differences in how motor areas are wired and how strongly the two hemispheres communicate, but these are trends and can’t be the basis of diagnosis. In other words, the brain shows differences that go with handedness yet there’s no scan or test that can predict handedness with certainty.

Second, handedness is partly genetic but it is polygenic, meaning it’s due to many genes, each with small effects. Development also matters. Small prenatal differences in growth or early brain organisation can tip a person towards one hand, even with similar genes. In many places social pressures have also reduced the prevalence of left-handedness because children are often trained to write with the right hand.

Finally, while being left-handed may bring some situational advantages — e.g. in direct combat or sports simply because it is rare — our everyday world is built for right-handers, from scissors and can-openers to school desks with right-handed writing arms, so why left-handedness has ‘organically’ stabilised in the population at around 10%.