We have seen a number of newspapers reporting recently on some research that was published in December 2018. As is often the case, the reporting is a bit sensationalist, drawing conclusions that are not really there in the underlying research results.
The report is headlined:
Babies who are bottle-fed are more likely to be left-handed! Whether you were breastfed or not is the key factor for dominant handedness in 20% of ‘lefties'
- Research carried out at the University of Washington School of Public Health
- Scientists studied 62,129 pairs of mothers and their babies in five countries
- Breastfeeding for six months decreased the chance of a ‘leftie' by up to 13%
- Around 22% of left-handers would be right handers if they had been breast fed
The researchers suggest that the process where the right side of the brain and left side of the brain take on specialist functions takes place up to nine months.
Philippe Hujoel, the study’s author said: ‘We think breastfeeding optimizes the process the brain undergoes when solidifying handedness. ‘That is important because it provides an independent line of evidence that breastfeeding may need to last six to nine months.’
Possible causes include the nutritional content of breast milk versus formula milk having an effect, or hormones produced in the baby when in contact with the breast.
This seemed a bit strange to us and we thought there was probably more to this than met the eye! So we asked two renowned experts and authors on left-handedness for their thoughts…
Chris McManus is Professor of Psychology and Medical Education, University College London and author of Right Hand, Left Hand.
Dear Keith, good to hear from you, as usual.
I think it is fair to say that there is little clear agreement on what might be the underlying mechanism, robust
As you say, most mothers are right-handed and there is a large literature showing that right-handers tend to hold babies on their left-hand side (for reasons which are also multiple, but typically that they want to keep their better hand free for feeding the baby). Not helped by left-handers mostly also holding babies on the left (and saying that they want to hold the baby with their better hand so that they don't drop it…); so not much agreement there at present.
Everyone may, therefore, be more likely to hold babies on the
Twins are of course a different story (and I can say from watching my wife with our twins that the only practical way is with both at the same time). But if that were the case then twins might perhaps be less likely to be left-handed than singletons, but the study also robustly shows, what we have known for a while, that twins are more likely to be left-handed than singletons.
So, yet another mystery about left- and right-handedness would be my overall summary!
Clare Porac is Professor of Psychology at Penn State University in Erie, PA, USA and author of “Laterality – Exploring the Enigma of Left-Handedness”.
Keith: There are two flags that cause concern. The researchers talk about a decrease in non-righthandedness. This is a catchall category that may indicate that the decrease is not in left-handedness but in everything other than strong right-handedness. The other flag is that the age at which handedness side is established. This is a point of controversy among researchers who study the development of handedness. The age range at which handedness is thought to be established goes from 1 year to about 3 years. It seems a stretch to argue that breast milk affects the motor areas of the right or left hemispheres in the first year of life and thus influences the side of handedness. The current view about handedness development is that it has multiple determinants only some of which may be biological.
The major claim of the press release is that as
The author acknowledges that he does not know what explains this finding and makes a weak attempt at trying to tie this to brain development. This press release is a good example of how weak results are amplified by the press to catch attention.
Clare is going to write an article on this for her own blog and we will link to it when it is published.
Additional comment from one of the original researchers
Nice to see how far and wide our articles get spread! We had a comment added by one of the original researchers behind the data used in the study above. Kevin Denny, Associate Professor at University College, Dublin, Ireland, said…
This paper arose, I think, as a response to my paper in Laterality 2012 (Breastfeeding predicts handedness). I showed fairly clear associations in two datasets (one UK, one Irish). Some Australian based researchers had come across it previously in a US dataset – which was got me interested. Hujoel largely replicates my finding as far as I know. Of course, neither he nor I know the mechanism but then we don’t know the mechanisms in general so I don’t see this as remarkable. As for effect sizes, while Chris says they are subtle I thought mine were substantial enough.