From Susan Gillingham
In your recent article on handedness, you mention that hand preference is widely supposed to be genetic, that humans are unusual in not having a 50-50 split of hand preference, and also the “right-shift factor” genes suggested by Marian Annett (5 November, p 36). I would like to suggest a contribution to this preference provided by simple selection pressure.
Due to the asymmetry of our digestive systems, gravity assists the digestive process of a baby lying on its left side, stomach or somewhere in between. The somewhere-in-between is very simply achieved by holding a baby with its neck and left shoulder in the crook of your left arm and your hand between its legs, so that the baby lies facing away from you and downwards with its chest and tummy along your forearm. This is a comfortable and secure hold for both baby and minder and will calm, in particular, a colicky baby.
A strongly right-handed mother will instinctively hold her baby in her left arm, leaving her dominant hand free for other tasks, so it would seem that right-handedness of a mother would be advantageous for both mother and baby. It wouldn’t be the first peculiar trait we have evolved as a result of the abnormally protracted infancy of our species.
Red Hill, Queensland, Australia
