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.

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.

Handedness & earnings

In this interview today on WEOL AM 930 (Cleveland, OH) I discuss "The economic consequences of being left-handed: some sinister results", co-authored with Vincent O Sullivan, Journal of Human Resources (2006), XLII (2), 353-374

Handedness & drinking behaviour

This paper has been published on-line in the British Journal of Health Psychology, print version to follow.

Handedness and drinking behaviour
Kevin Denny

Objectives: A number of papers have investigated whether there is an association between handedness and alcohol consumption hypothesizing that alcoholism may be a consequence of atypical cerebral lateralization or a response to the stress involved in being a minority in a right-handed world. Research to date has mostly used small clinical samples, some without a comparison group. This paper exams this issue using a large population-based random sample.

Design: A large multi-country data set of nationally representative samples of the non-institutional population aged 50 years and older from 12 European countries was used (N=27,428). Methods Logistic regression was used to model the frequency with which individuals self-report the frequency of alcohol consumption. A series of models with differing numbers of potential confounders are estimated. The predictors of frequent and infrequent drinking are investigated separately.

Results: After controlling for a number of confounders it is shown that left-handers do drink more often. However, this is due to them being less likely to drink rarely (less than once a month) or not at all.

Conclusions: The evidence suggests that while there is an association between left-handedness and frequency of alcohol consumption there is no reason to believe that it is associated with excessive alcohol consumption or risky drinking.

More love advice from Dr Kev

I have been working on laterality, specifically handedness, for some time. But there is a lot more to laterality than handedness and lateralisation in the brain is an important topic in neuroscience. The following paper caught my eye as being potentially useful to my young, or even not so young, colleagues.
What McKay et al show is that self-esteem is lateralized.So imagine subjects hearing particular words in each ear (i.e. one ear at a time). It turns out that words heard via the right ear (which are processed in the left-hemisphere) generate a higher feeling of self-esteem than those heard via the left-ear. So if you are going to whisper sweet nothings to your partner, or your desired partner, then you know what to do. I think you will be pleased with the results.
Alternatively, if you want to castigate your PhD student for their general fecklessness, use the left-ear if possible. As far as I can recall, the sense of smell is also lateralized 'though this may be harder to take advantage of in practice.

McKay, R., Arciuli, J., Atkinson, A., Bennett, E., & Pheils, E. (2010). Lateralisation of self-esteem: An investigation using a dichotically presented auditory adaptation of the Implicit Association Test. Cortex, 46 (3), 367-373
http://dx.doi.org/10.1016/j.cortex.2009.05.004

Returns to footedness

Its good to see that economists' continuing interest in laterality. This paper does not investigate left vs. right-footedness but measures the benefit of being "two-footed". Some work of mine (Neuropsychologia , 2008, 2091-2094) shows how ambidexterity conveys certain cognitive advantages.

The returns to scarce talent: footedness and player remuneration in European soccer

Alex Bryson, Bernd Frick and Rob Simmons, CEP Paper No CEPDP0948

Abstract: We investigate the salary returns to the ability to play football with both feet. The majority of footballers are predominantly right footed. Using two data sets, a cross-section of footballers in the five main European leagues and a panel of players in the German Bundesliga, we find robust evidence of a substantial salary premium for two-footed ability, even after controlling for available player performance measures. We assess how this premium varies across the salary distribution and by player position.

http://cep.lse.ac.uk/_new/publications/abstract.asp?index=3465