Breastfeeding and handedness

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.

Graph of the year? Mother's education and breastfeeding

Its that time when people reflect on the year gone by, lists of pivotal events, dead celebs etc. There are also lots of suggestions for "graph of the year" going round. It's pretty arbitrary of course since it depends on what people find interesting. Graphs are great but they have the potential to mislead since, by their nature, they are simple but evocative.
Anyway, here is one I find very striking. It graphs the rate of breastfeeding initiation (i.e. whether the child is breastfed at all) by their mother's education level using the Growing Up in Ireland data. It is one of the most pronounced socio-economic gradients I have come across. A child whose mother is a graduate has about a 77% chance of being breastfed. At the other end of the scale it is a staggering 17%. If, instead of looking at "ever" you look at a duration of "6 weeks+" the story is much the same.
Note that the average (i.e. the % ever breastfed, using sampling weights) is about 45% which is way lower than pretty much anywhere else I have looked at. The US and UK data are in the low 70's. Many countries are higher. Why there is such a low average and such a steep socio-economic gradient in Ireland I don't know. But given the benefits to mother and child it should be a matter for concern - and action.

Socio-economic predictors of breastfeeding in Ireland

When a child is born the first investment decision a parent makes is whether to breastfed the child. This usually occurs within the first hour. Of course decisions taken beforehand, like diet, also have an important role. Since breastfeeding is generally considered to have important physical and other benefits for both child and mother, understanding who is more likely to breastfeed (& ultimately why) is important. This is particularly the case in Ireland where breastfeeding has a relatively low incidence.
It might be conjectured that social class has some role to play. High SES mothers may have access to better information (or be better able to process the information) as well as support networks. They have less of a need to return to work. On the other hand, the opportunity cost of their time may be higher.
The graph below shows the proportion of children breastfed by socio-economic class (where there are two adults in the household, it is the highest of the two classes).

There is a very striking gradient with children from manual backgrounds being much less likely to be breastfed.
Of course, one can't be sure what the reason for this link is. Class is correlated with education, income and other factors. Looking at the gradient with respect to the primary carer's education one finds:

Again there is a strong pattern with children of more educated mothers being much likely to be breastfed. What happens if we put both factors together in a model? Well both play an independent role although education seems to be the dominant factor. Interesting, household income does not have a significant role to play given social class & maternal education.
One can find other predictive factors: younger mothers and those from rural areas are less likely to breastfed. But these factors seem to have a fairly small role compared to social class and education.
If we look at mother's behaviour during pregnancy for clues as to whether they breastfed one comes across an interesting anomaly. Those who smoked during pregnancy are about 9% less likely to breastfeed, other things being equal, whereas those who consumed alcohol are about 3% more likely.

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.