Scientists think they have found the reason why some people are left-handed — and it has nothing to do with the brain. There have been a few theories over the decades about why some people are left-handed. 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. The research — by Sebastian Ocklenburg, Judith Schmitz, and Onur Gunturkun from Ruhr University Bochum, along with other colleagues from the Netherlands and South Africa — found that gene activity in the spinal cord was asymmetrical in the womb and could be what causes a person to be left- or right-handed. Arm and hand movements start in the brain, in an area called the motor cortex, which sends a signal to the spinal cord that’s translated into a motion. The researchers found that while the fetus is growing in the womb, up until about 15 weeks, the motor cortex and the spinal cord are not yet connected, but right- or left-handedness has already been determined. In other words, the fetus can already start movements and chooses a favorite hand before the brain starts controlling the body.