There are many studies of the effects of breastfeeding on the health of babies (and their mothers). There are also numerous theories about the causes of left-handedness. Many of the latter have not found strong empirical support although it seems clear that handedness is heritable. A connection between these phenomena has not hitherto been noticed.
In a paper, with the above title & forthcoming in Laterality, I show using data for Ireland (GUI) and the UK (NCDS) that children who have been breastfed for more than a minimum of 4-6 weeks are significantly less likely to be left-handed. Abstract below:
This study examines whether being breastfed predicts handedness. Two large representative samples from Britain (n=13,421) and Ireland (n=8,426) are used. Multivariate probit models are estimated, reporting the marginal effects of covariates on the probability of a child being right-handed. For both datasets, children who have been breastfed for a minimum period (between one month and six weeks) are significantly less likely to be left-handed. The results are robust to a variety of controls. Hence breastfeeding is an environmental factor that predicts handedness in a non-linear way although the mechanism is unknown.