Breastfeeding predicts handedness

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.

Handedness and ability at maths: evidence from Ireland

There is a great deal of interest both popularly and amongst scholars about whether cognitive ability is predicted by handedness. The literature contains many findings which cannot be simply summarized and there are many many myths. Evidence for Ireland has been non-existent, as far as I am aware, until now with the release of the Growing Up in Ireland data.
So what can we say? Below I plot the density of attainment at a maths test that the 8 year olds in GUI sat.
Sadly, if you are a ciotóg, you can see the distribution is shifted to the left - but not by much. The good news is that when you look at the distribution of reading ability, there is no difference at all.
In numerical terms, left-handers are about 8% of a standard deviation lower. By comparison girls are about 11.5% of one standard deviation lower.