Right or left-handed? It’s decided in the womb, say scientists

Tony Jones

It had been thought that developing a favoured hand was post-natal and decided by the age of three or four.

But a new study has found foetuses showing a preference for one side over the other usually retain it when after they are born.

The research was carried out by a team led by Peter Hepper of the Foetal Behaviour Research Centre at Queen’s University, Belfast, who studied ultrasound scans of 1,000 foetuses.

In one study of 274 foetuses at 15 weeks’ gestation, nine out of 10 preferred to suck their right thumbs, New Scientist reported.

Mr Hepper’s team followed 75 of those foetuses after birth, and found that at 10 to 12-years-old, all 60 of the right thumb-suckers were right-handed, while 10 of the 15 left thumb-suckers were left-handed and the rest right-handed.

At 10 weeks old, even before they suck their thumbs, foetuses wave their arms.

A second study found that most prefer to wave their right arm, a preference that persisted until 24 weeks, after which the foetus is too cramped to move.

Mr Hepper stressed the observations do not show the foetus can control its movements so early.

Nervous connections to the body from the brain are not thought to start developing until around 20 weeks’ gestation.

“There is no evidence that the brain has any control over these movements at this stage,” said Mr Hepper.

“It’s most likely to be a local reflex arc involving the spinal cord.”

He speculates that a foetus may have a preference for one side of its body simply because that side has developed slightly faster.

The previous theory on how right or left handedness develops said it was a side effect of brain lateralisation - in which one side of the brain predominately handles certain tasks like language.