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Are breastfed babies less likely to be left-handed?

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 though the statistical relationship is to breastfeeding.  So it could be mediated via a whole host of possible factors which are correlated with breast-feeding.

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 left-side to start with, but that probably won't last long as there are two breasts, and mothers know that they should be used equally, for reasons not least of comfort. So the net result is probably roughly equal amounts of feeding on both sides, or visible maternal effects would probably result.

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.

All in all I think we are left with some very secure statistical relationships which replicate very well, but the effects are still quite subtle in size, and I would say that at present we simply have no proper theoretical analysis of what is going on. My guess is that breast-milk is an important nutrient in early life (that is why nature provides it), and there are complex fats/amino-acids which benefit the developing brain and that impacts how the two brain sides develop and mature (and infants' brains are maturing very quickly in the first 6-9 months of life). But exploring that and proving it will be far from easy.

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 breast-feeding duration increases left-handedness decreases, however, this is true for only some of the studies. This study is not original research but a reanalysis of 6 existing databases. Some, not all, of these studies show the relationship between handedness and breastfeeding.

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.
Kevin Denny

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25 comments on “Are breastfed babies less likely to be left-handed?
  1. Laura F says:

    My Mum and aunt had 4 girls between them… 3 of the 4 of us are left handed (but none of our parents & grandparents are) and all of us were breastfed, so no correlation here!

  2. Diana Walch says:

    I am a left-handed identical twin, my sister was right -handed. I had 3 children all left-handed.They were breast fed for the first 4 months and then bottle fed with whole milk until 12 months old

  3. cathi easter says:

    I’m a left hander, as are both my girls who were breast fed until they were a year old. Approximately 30% of my family on my mother’s side are left handed, so are my birth father and my adopted dad. There seems to be a strong genetic link to left handedness in my family.

  4. cathi easter says:

    I am left handed as are both of my daughters, who were breast fed until they were a year old. Approximately 30% of my family on my mother’s side are left handed. My birth father and my adopted dad are both left handed too. There seems to be a strong genetic link to left handedness in my family.

  5. Enid Cruickshank says:

    Interesting! I’m left-handed, breastfed till I was at least a year old, mother right-handed. All 3 of my children were bottlefed and I would hold them against my right side when feeding them. All are left-handed or as ambidextrous as all lefties have to be in a right handed world.

  6. Ben says:

    I don’t agree at all with this…I think it has more to do with the genetic makeup… I am a lefty, as was my father, his sister,and several cousins in my family, from both dad’s side and mom’s…and we were all breast feed….My son was bottled fed from both and is right handed…as are several friends and family…I believe further real studies need to be made before coming to any real conclusions..Just my input…

  7. Mary Nasvik says:

    I am one of six kids who were breast fed for 3 months then were switched to the bottle. Five of us are left handed, and one is right handed. How does this fit into the research?


    Our family is enormous I’m the only left handed child & my own children are also right handed. We have all been breastfeed I’ve never heard such aload of rubbish. I wish there were more of us!!!

  9. Glenys Craig says:

    I have 4 children, 2 boys & 2 girls, all breast-fed for from 9 to 13 months. Boys both right-handed, girls both left. One girl breast-fed for 9 months, one for 13, boys in between. I’m left-handed, my husband right. Husband and boys O blood groups, all the females A. You could draw all sorts of conclusions from those even male-female splits I think! I imagine left-handedness is much more complex than how long a child has been breast-fed.

  10. Maggie says:

    Interesting… Both my kids breastfed to two or beyond. I’m a very strong lefty. I was actually expecting my son to be a lefty as well but he’s either ambidextrous or right handed. He’s almost 5 and hasn’t picked a hand yet. His dad and I are both lefties, both his half sister’s from his dad are lefties and so is his grandma on that side.

  11. Chris Meier says:

    My breastfed daugther is left-handed. I’m right handed ! But could well have been converted as this was still common when I was a kid. So I’m quite good at using my left hand for things that feel better, like putting on make up on my left eye !

  12. Laura F says:

    My mum and her sister had 4 girls between them… all 4 breastfed (for varying lengths of time), 3 of the 4 are left-handed, so no correlation in my experience!!

  13. Julie says:

    If this had a strong correlation and causation, wouldn’t we be seeing an increase in lefthandedness in the population that aligns with the decrease of women breastfeeding at all or for a reduced time frame? I’ve heard nothing about a change in the percent of the population that is left handed. Though I’m not sure how that data is even tracked as I have no memory of putting down on offical documents what hand I use.

  14. Judy says:

    My Mother breast fed both my Brother and me. I am (very) left handed. My brother was showing tendencies of being left handed; however, our Grandmother was living w/our parents at the time and she thought left handed was an evil curse so she changed him. It didn’t seem to cause any “emotional” problems in my brother but, he did show some of the side effects we already know about today. (1) He was above-average intelligence; however, he was insecure with spelling; and (2) 80% of the time he would print everything rather than write “cursive.” My Grandmother died when I was 9 months old so she was not around to “change” me from left to right.

  15. Kaitlin Smith says:

    Anecdotally, I was bottle-fed exclusively and I am left-handed.

  16. Suzanne Nelson says:

    I am left handed. My mother in law is left handed. My husband and I thought 1 of our 3 kids may be left handed but no. Asll 3 were breastfed. First one weaned at 14 months. She is slightly ambidextrious. Number 2 weaned at 2 yrs and #3 at 3 years (was just a nitetime thing for last year ). 2nd and 3rd kids are very creative. All intelligent. First one nearly genius. First one showed some slight left hand preference but my husband then started handing things more directly to her right hand.

  17. Sven T says:

    Both of my kids (son and daughter) were breastfed (by a righthanded mother). Both kids lefthanded. I’m lefthanded. I was breastfed by my righthanded mother. My father was lefthanded. Don’t know about grandparents breastfeeding :-).

  18. Judith Lloyd says:

    Do not believe this survey at all. I think that as a small percent of folks are gay, some are redheads and some of us are lefties. Just God’s plan that we all are individuals, different from everyone else, with many things that we can’t explain, but should enjoy and love everyone’s unique differences

  19. Lyelle Palmer, Ph.D. says:

    In the Stimulating Maturity through Accelerated Readiness Training teacher training at the Minneapolis non-profit A Chance to Grow, Inc,, brain dominance is the focus. Therefore knowing which hemisphere is the language side for handedness and sighting dominance is the issue determining the best skilled hand. The test easily identifies the spurious lefties through a speedy, automatic response to the command: “As fast as you can, write your name on the floor with your foot, go! Since children are never taught to write with their feet the reaction is spontaneous/automatic without cortical interference or distraction through thinking about a decision that is often confused with left=handers. Foot dominance is useful because it is valid and efficient in the classroom and has practical application immediately. Teachers can test many or several children at the same time. Do not tell the students the purpose before the test; emphasize speed. Occasionally a child will begin with one foot and finish with the other and using the right hand is always acceptable with an indecisive result. Eyedness for accuracy is another matter. Sighting with both eyes open by pointing at a distant target (the nose of the teacher or examiner) will show the eye that will determine success or failure in sports and games. Cross dominance is when the right hand is guided by the left eye and an object thrown will be off target and a source of social embarrassment (self-doubt, and sometimes shame and ridicule). Often the cross dominance is the result of the inability to wink one eye and training is easily completed over several weeks by holding down the eyelid of the winnable eye and closing and opening the non-wining eyelid 20 times per day. True left handers should use the left eye for sighting. Tube sighting/watching of TV through the desired eye with both eyes open is also indicated when retraining the alignment of eye and hand coordination. Each person can demonstrate personally the difference between sighting the fingertip on a target when one or the other eye is covered. The difference indicates how far off target the aim will be when cross dominant. My wife was unable to knock down any pin while taking a college bowling class and refused ever again to go bowling because of embarrassment. The development of basic coordination can be accomplished in early childhood so that accuracy builds confidence and leads to participation and contribution in team sports. Ignoring or making excuses for cross dominance issues has lifelong consequences, but many adults who discover the condition can compensate or retrain. In WWII 40,000 marines were retrained to sight with their right eyes. If all else fails, an optometrist can prescribe for a dominance lenses in which the vision in one eye is enhances while the other eye is suppressed (the dominance changes immediately).
    Lyelle Palmer, Ph.D.

  20. Sandi Pitsenbarger says:

    My mother brestfed all 4 of us kids, & 3 out of the 4 are left handed.

  21. Mike Ross says:

    I’d be interested in learning more about genetic aspects of left-handedness.

    For several years before we had children, I’d been aware of a number of studies that purported to result in a child’s handedness, including 1) right-handed parents usually offering items to a child’s right hand and 2) children who sleep on their stomachs turn their heads to look at their preferred hand.

    For both of my children, I made intentional efforts to offer left-handed opportunities — such as giving items to their left hand and positioning their heads so they’d look at their left hands — and they both demonstrated resolutely right-handed preferences: they’d reach with their right hands for the offered items and would turn their heads to look at their right hands.

    They both were breast-fed … so maybe that’s a real factor? Hard to imagine, given the breast switching mentioned above.

  22. Mrs. Lucille Burns says:

    Daughter allergic to my milk; took awhile to find formula she could tolerate. Therefore, went right to formula with son. Both held in my right arm when fed.
    Diapers changed, baths given all in same direction.
    My father left-handed. I, the latest of 3, also.

  23. Karen Mader says:

    Thanks for all the info!!!!! Glad that you looked into it. Thought someone might say that there are more left-handed people now because less are converted to using their right hand.

    • kevindenny says:

      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.
      Kevin Denny

  24. Gloria Tamura says:

    I think that one way to shed light on the breastfed/handedness topic would be to know about handedness in the past, long before women were able to buy formula. Do we know the statistics for handedness in past centuries? We could assume that all or most babies were breastfed, either by the mother or a wet-nurse. What were the handedness ratios then? I do not know the history of safe breast milk substitutes but I am guessing that safe formula is a somewhat recent invention.
    And of course, the mother favoring her own handedness in using one beast more than the other is laughable. Of course she switches from one to the other. The breasts swell and are painful and leak if not emptied. The baby determines the amount of milk that the breasts produce; the milk supply adjusts to the amount of milk the baby drinks, increasing the milk as the stimulation increases and as the baby grows and requires more milk.

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