Left-handed people haven’t always been treated well throughout history. They’ve been persecuted for their disposition, being been labeled as evil — or even as witches — despite making up about 10% of the population. In fact, the word “sinister” comes from “left” or “left hand.”

There have been a few theories over the decades about why some people are left-handed, including an outdated idea that it has something to do with mothers who are stressed while pregnant.

It’s down to the spinal cord — not the brain

Research since the 1980s has found that our preference for our left or right hand is most likely determined before we are born — ultrasound screenings suggest as early as the eighth week of pregnancy. From the 13th week in the womb, babies tend to suck either their right or their left thumb.

It was previously thought that the genetic differences between the left and right hemispheres of the brain determine whether someone is left- or right-handed. But a study published last year in the journal eLife found that the answer could lie in the spinal cord.

These gene-expression differences could affect the right and left parts of the spinal cord differently, resulting in lefties and righties.

So why are lefties so rare?

Scientists have long tried to answer this.

In 2012, researchers at Northwestern University developed a mathematical model to show that the percentage of left-handed people was a result of human evolution — specifically, a balance of cooperation and competition.

In other words, they thought that, though the basis for right- or left-handedness may be genetic, there could be a social factor that explains why the ratio is so high.

“The more social the animal — where cooperation is highly valued — the more the general population will trend toward one side,” Daniel Abrams, an assistant professor at the McCormick School of Engineering and Applied Science who helped develop the model, told LiveScience.

“Thus, the right eye is stimulated by light before hatch, whereas the left one is mostly light deprived. This combination of genetic and environmental factors (light) induces a visual asymmetry — pigeons and chicken are better in visual discrimination, categorization, and memorization of visual patterns with their right eye than with their left eye. If chicken or pigeon eggs are incubated in darkness, the development of this asymmetry is prevented.”

Why exactly people are left-handed is still a bit of a mystery — partly because left-handed people are often excluded from scientific research, experts say — and it’s hard to predict whether a child will be left or right-handed once they are born.

One thing we do know, though, is that the neurological differences between left- and right-handed people are small, and supposed behavioral or psychological distinctions have largely been debunked.