Science / Medicine : Left-Handedness Seed in Womb

A study of thumb-sucking among fetuses shows the tendency to be left-handed may start in the womb, Irish researchers reported last week in the British journal Nature.

About 10% of the adult population is left-handed, and scientists have long debated whether that preference stems from genetic factors or is caused by some kind of injury, perhaps occurring during birth. Now, the researchers said they have found evidence some fetuses display a preference for sucking their left thumbs as early as 15 weeks after conception.

Using ultrasound to examine the fetuses of 224 women who had normal pregnancies, the researchers found 12 of the 224 fetuses--or about 5.4%--favored their left thumb. The positions of the fetus in the womb had no effect on thumb preference.

Follow-up observations of 17 fetuses indicated “preference for a particular thumb is maintained during pregnancy,” the researchers said. The discovery, they said, “demonstrates for the first time the existence of behavioral asymmetries before birth” and “supports genetic explanations of handedness.”