The causes of handedness, and why a small proportion (about 10%) of people turn out to be left-handed, are not well understood. It is known that handedness is heritable: having a left-handed parent (or two) increases the probability of a child being left-handed. There has been some progress in understanding the genetics in recent years. There are various other theories about environmental influences. One such theory, associated with Bakan, suggests that difficulties during pregnancy or birth can increase the probability of being lefthanded, although I am not sure the evidence for this “pathological left-handedness” is that strong. There is also evidence that very low birthweight and a high birth order (say 4 or more older siblings) can have the same effect.
A connection with breastfeeding has not hitherto been studied. In a paper Breastfeeding predicts handedness, just published in Laterality, I show that children who were breastfed for a minimum of around a month to six weeks are significantly more likely to be right-handed – by between 1 and 3 percentage points. Since one expects about 10% to be left-handed this is quite large. This is shown for the UK (with the National Child Development Survey) and Ireland (Growing Up in Ireland). The results are robust to a range of controls. It is generally believed that breastfed children have slightly higher cognitive ability. The mechanism behind this is not understood: it could obviously be something in the milk or it could be the actual act of breastfeeding. There is evidence from animal studies that the quality of parenting can affect the brain’s development. I speculate that similar mechanisms might be behind the connection with handedness.
It would be interesting to look at children who were deprived of this early nurturing experience (for example those put into orphanages) to see if a similar pattern existed. One could also look at those mothers who experienced famines while the child was in utero to see if deficits in ante-natal nutrition had an effect.
Interestingly, in the Irish data, there is a slightly higher proportion of left-handed children than normal (just over 13%) and this might be partly explained by the unusually low incidence of breastfeeding in Ireland.