It is not the brain that determines whether a person is a lefty or a righty, but the spinal cord, a study has claimed.

Until now, it had been assumed that differences in gene activity of the right and left hemisphere might be responsible for a person's handedness -- people's tendency to naturally favour the use of one hand over the other. 

But the recent study demonstrated that gene activity in the spinal cord is asymmetrical already in the womb and could be linked to the handedness of a person.

"Our findings suggest that molecular mechanisms for epigenetic regulation within the spinal cord constitute the starting point for handedness, implying a fundamental shift in our understanding of the ontogenesis of hemispheric asymmetries in humans," said Sebastian Ocklenburg from Ruhr University Bochum in Germany.

According to ultrasound scans carried out in the 1980s, a preference for moving the left or right hand develops in the womb from the eighth week of pregnancy. From the 13th week of pregnancy, unborn children prefer to suck either their right or their left thumb.

Arm and hand movements are initiated via the motor cortex in the brain. It sends a corresponding signal to the spinal cord, which in turn translates the command into a motion. 

However, the motor cortex is not connected to the spinal cord from the beginning. In fact, even before the earliest indications of hand preference appear, the spinal cord has not yet formed a connection with the brain, stated researchers in the paper appearing in the journal eLife.

In addition, environmental factors were found to be controlling whether spinal cord activity was greater on the left or right side.

For the study, the team analysed the gene expression in the spinal cord during the eighth to 12th week of pregnancy and detected marked right-left differences in the eighth week -- in precisely those spinal cord segments that control the movements of arms and legs.