International Left-Handers Day: Why Are Some People Left-Handed?

A left-handed child writes on August 23, 2015. DIARMID COURREGES/AFP/Getty Images

Today, is International Left-Handers Day, an annual celebration of this strange quirk of human biology which affects around 10 percent of the world's population. But why are some people left-handed?

Several studies over the past few decades have shown that your hand preference appears to be determined before birth. One paper, for example, published in the journal Scientific Reports, suggests that this trait is well-defined by the 18th week of gestation, while others show that by 13 weeks, unborn children prefer to suck their right or left thumb.

But the science behind why people become lefties remains inconclusive. It has long been thought that genetic differences between the brain's left and right hemisphere were the main reason. However, some recent research has begun to offer alternative explanations.

For example, a study published in the journal eLife in 2017, suggested that preferences for the left or right hand can be traced to "asymmetry" in the spinal cord.

Arm and hand movements are controlled by the motor cortex in the brain, which sends a signal to the spinal cord, which in turn translates that command into motion. The motor cortex, however, is not connected to the spinal cord from the beginning. Even before this connection forms, signs of hand preference become apparent, suggesting that the root lies in the spinal cord rather than in the brain.

Meanwhile, other research from human geneticist Silvia Paracchini at the University of St. Andrews in Scotland has also focused on asymmetry in the body in a bid to understand why right-handedness is so much more common.

"We look symmetric, but if we look inside at our organs there's lots of asymmetry," she said in a speech to the Royal Society of London in 2015.

Paracchini and her colleagues investigated a rare condition known as situs inversus in which the major organs are reversed from their normal positions—for example, the heart would be on the right side of the body.

The scientists found that elements of the abnormal genetic code in people with this condition also affected whether people were left- or right-handed.