Breastfeeding and Reading ability: some Irish data

The data from Growing up in Ireland is becoming available for secondary analysis. It is a great resource which, hopefully, researchers in Ireland and elsewhere will take full advantage of. From time to time I will post some snapshots from the data.
The graph below shows the distribution of reading ability (at age 9) by whether the child was breastfed. Its clear that the distribution for those who were breastfed is to the right: the average is about one third of a standard deviation higher. The proportion who were breastfed is 51%, low by international standards.
The graph does not imply or assume anything about causality: it could be that this relationship is due to confounding, for example socio-economic status and parental education is strongly positively associated with the decision to breastfeed. In general though the evidence appears to suggest some positive effect of breastfeeding on cognitive ability. There is extensive research on this topic including work in progress at the Geary Institute by myself and Orla Doyle.