Breastfeeding increases right-handedness? Yet another example of misusing data to make insupportable claims about breastfeeding.

Left-handed man shows a dirty hand after writing mockup

No sooner did I finish writing a post reviewing the large and growing body of evidence that the benefits of breastfeeding have been exaggerated out of all proportion to the data, another researcher comes along making another unsupportable and absurd claim.

I’m glad because it can serve as an object lesson and in how and why breastfeeding has been promoted beyond reason, sometimes to the detriment of the physical health of babies and the mental health of mothers.

[pullquote align=”right” cite=”” link=”” color=”” class=”” size=””]Nearly all babies born in industrialized countries between 1955-1975 when formula use peaked were left-handed! Wait, what? They weren’t?[/pullquote]

The paper is Breastfeeding and handedness: a systematic review and meta-analysis of individual participant data published in the journal Laterality: Asymmetries of Body, Brain and Cognition.

Publicized by the author’s institution, it has received widespread public notice.

Want a right-handed baby? Breastfeeding may play a role, study finds is typical.

New parents might not be able to control a baby’s sleep habits or crying, but new research suggests mothers could play a role in determining if an infant becomes right-handed or left-handed.

Babies who are breastfed are less likely to be left-handed compared to infants who were raised on baby formula, University of Washington researchers found. Their study analyzed more than 60,000 mother-child pairings, and took into account other factors linked to handedness, making sure the research zeroed in on the link with breastfeeding in particular.

What did the actual study show? According to the author:

In summary, the findings of this study offer an independent line of evidence that breastfeeding may enhance brain lateralization. This finding provides additional evidence to counter the arguments of those whom dismiss breastfeeding despite its evolutionary normality, and despite the precautionary principle…

Now we understand why nearly all babies born in industrialized countries between 1955-1975 when formula use peaked were left-handed!

Wait, what! They weren’t left-handed? The incidence of left-handedness didn’t change at all?

What’s going on here?

The author gives us an important clue. He reveals that white hat bias — bias in the service of perceived righteous ends — drove him to respond to those who dismiss breastfeeding has having limited benefits by “finding” a new benefit. It seems never to have occurred to him that there is no reason to conduct a systematic analysis and meta-analysis when we already have the population data on what actually happened. And he was so fixated on finding a benefit that he ignored reality: large scale use of formula has had no impact on handedness.

Handedness seems to have a strong genetic component.

Two right-handed parents produce fewer left-handed offspring than parents with any other handedness combination and two left-handed parents produce the highest proportion of left-handed children, i.e. approximately 30–40%…

Handedness is observable is archeological evidence:

Handedness in ancient humans has been inferred by analysis of archaeological samples from skeletons, stone tools and various other artefacts (see Steele & Uomini (2005) for a review). By studying arm bone length, Trinkaus et al. (1994) observed a prevalence of right hand dominance in Neanderthal skeleton samples (dating from approx. 35 000 BP)…

With the advent of ultrasound, handedness can be detected before birth:

At 9–10 weeks, the foetus begins to exhibit single arm movements: a majority (75%, n=72) exhibited a greater number of right arm movements, 12.5 per cent a greater number of left arm movements, and 12.5 per cent an equal number of left and right arm movements… From 15 weeks of gestation, the foetus exhibits a preference for sucking its right thumb and the sucking behaviour at foetal state is related to hand preference at a later age…

Handedness, or rather expression of handedness, is culturally mediated. According to the author of Laterality: Exploring the enigma of left-handedness:

The highest rates of left-handedness, 10%, occur in North America, Australia, New Zealand and western Europe. The lowest rates, 4 to 6%, are in Asia, Africa and South America…

… Countries with higher rates of left-handedness are societies that tolerate left-handedness, do not punish left-hand use and do not pressure left-handers to convert to right-handedness…

How has handedness in the US changed over time?


Left-handedness fell in the late 1800’s to a nadir around 1910 and then rose steadily until 1960. Formula feeding rates were rising during part of that time, but then they began to fall precipitously and there has been no drop off in the incidence of left-handedness suggesting that formula feeding has no impact on handedness.

The disconnect between the data that the author analyzed and what has actually happened with handedness over the same period of time is similar to that found in much of the literature purporting to show benefits of breastfeeding. For example, various authors have claimed that by increasing breastfeeding rates we could save thousands of infants lives. But population data involving tens of millions of babies gathered over decades during which breastfeeding rates fell precipitously and then rose precipitously show that the predicted saving of lives did not occur. The theory has been utterly disproven by reality, yet breastfeeding researchers (and lactation professionals) still cling to the theory and ignore reality.

That’s why when you read about various claimed benefits for breastfeeding you should ask what has actually happened over time. We don’t have to rely on mathematical models of what was supposed to happen when we have the data on what actually happened.

That the author of this paper on handedness made no attempt to test his conclusion against reality is typical of breastfeeding research. All claims of breastfeeding’s benefits should be viewed with a great deal of skepticism unless and until they comport with what actually happens in the real world. Otherwise we risks being duped by researchers whose primary goal is to “counter the arguments of those whom dismiss breastfeeding despite its evolutionary normality.”

57 Responses to “Breastfeeding increases right-handedness? Yet another example of misusing data to make insupportable claims about breastfeeding.”

  1. namaste
    January 10, 2019 at 12:47 am #

    well for chrissakes, what’s wrong with being left handed?

    • kilda
      January 10, 2019 at 10:07 am #

      I’ve actually been sitting here trying to think of a more meaningless “benefit” that breastfeeding could confer than handedness, and I really can’t.

    • sdsures
      January 10, 2019 at 2:04 pm #

      As a knight of the realm, will it be easier to stab my enemies with my broadsword in my left hand?

  2. The Bofa on the Sofa
    January 9, 2019 at 2:50 pm #

    As a father, I will just point out that that there are advantages to being left-handed in both baseball and volleyball….

    I wish my kids were left-handed. Unfortunately, despite my best efforts to persuade them, they are not….

  3. Heidi
    January 9, 2019 at 12:08 pm #

    I write with my left-hand although I consider myself ambidextrous just because some things I prefer to do right-handed. I suspect that I had to adapt in a right-handed world. I think I used to do everything left-handed as a kid. I’m not sure I see issue with being left-handed. If breastfeeding was correlated with left-handedness, it would be spin into a positive.

    • The Bofa on the Sofa
      January 9, 2019 at 2:49 pm #

      I’d give my right arm to be ambidextrous….

      -Yogi Berra

  4. Zuul
    January 9, 2019 at 10:16 am #

    So, I combo fed. Is that the reason my kid is ambidextrous?

    And here, all this time, I thought it was neat genetic quirk, because both of my brothers are ambidextrous.

    So I guess if you want your kid to be able to perform the neat party trick of writing with both hands, feed formula AND breastmilk.


    • Zuul
      January 9, 2019 at 4:16 pm #

      Haha, I just talked to my mom, and she says she combo-fed my brothers, too. I was exclusively breast fed and I’m so right-hand dominant that I can hardly accomplish anything with my left.

      So far we have a sample size of four. Is that enough to publish yet?

  5. rational thinker
    January 9, 2019 at 7:34 am #

    These newfound benefits of breastfeeding get more ridiculous every time. Watch next they will say breastmilk cures pink eye, just have to put a teaspoon in eye and you will be cured. Or perhaps if you inject the breastmilk into a cancerous tumor it will destroy the cancer and you will be cured!

    • Hannah
      January 9, 2019 at 8:14 am #

      I’m sure you meant to be ironic but that’s what they told me, put breastmilk into your baby’s pink eye, breastmilk is sterile and overall beneficial. I decided to smile, nod and see a doctor.

      • January 9, 2019 at 8:50 am #

        Breastmilk as a “cure” for pinkeye is popular among the woo crowd. My neighbor who gives her kids amber teething necklaces and only buys organic recommends it. The thought that squirting a sugary substance into your eye is going to fix it is bizarre. (I should add that my neighbor vaccinates her kids and gets them regular medical and dental care, and she had her kids in the “natural birth” wing of our hospital–so she’s not crazy, just crunchy.)

      • sdsures
        January 9, 2019 at 8:55 pm #

        Bugs love sugar, and milk – ANY type of milk – has sugars in it.

        How come crunchy people don’t get this basic chemistry fact?

        • Hannah
          January 10, 2019 at 1:24 pm #

          That’s what I was thinking when my midwife recommended to put some breastmilk on my babies belly button. It kept bleeding for a couple of days after the umbilical stump fell off. A mixture of blood and milk on an open wound, seriously?

        • swbarnes2
          January 10, 2019 at 2:37 pm #

          I think breastmilk might be effective at helping blocked tear ducts, but infection is obviously different.

      • January 10, 2019 at 4:59 am #

        URine is sterile, too [normally]

        • Hannah
          January 10, 2019 at 1:27 pm #

          Have you not heard of all the fabulous benefits of drinking morning urine?

          • Cristina
            January 11, 2019 at 4:27 pm #

            That’s for amateurs. You have to put it in your eyes after letting it sit for a few days first! I really wish I didn’t know as much about this as I apparently do. I blame Facebook.

        • Eater of Worlds
          January 16, 2019 at 4:24 pm #

          Urine is actually not sterile.

    • kilda
      January 9, 2019 at 11:05 am #

      don’t be silly, as long as you breastfeed no one will ever get cancer. Unless they vaccinate, of course.

    • Angharad
      January 9, 2019 at 3:27 pm #

      I was told that breastmilk cures eczema and to just rub some on my baby’s cheeks and it would help her. I thought it couldn’t hurt, and two days later was in the pediatrician’s office with a tiny infant with impetigo. I still feel really bad about it four years later, and probably should have seen the obvious issues with it, but I was exhausted, desperate, and hormonal.

      • Zuul
        January 9, 2019 at 4:33 pm #

        You know, it’s funny, my kid had all kinds of rashes and eczema when she was breastfed. When I started supplementing with formula, she started to get better. When I switched to formula entirely, they went away and she hasn’t had any skin problems since. So much for the pure cure-all of breastmilk.

        • Angharad
          January 9, 2019 at 4:48 pm #

          Oh, yeah, it turned out that her eczema was caused by allergies to various proteins in my breastmilk. Once I switched her to a diet of Nutramigen, she fully recovered. So I really was unintentionally adding insult to injury by rubbing something she was allergic to onto the evidence of her allergies. :

          • PeggySue
            January 9, 2019 at 7:27 pm #

            What an awful thing for you to go through. Glad you took her to the doctor, but stressful.

    • PeggySue
      January 9, 2019 at 4:00 pm #

      Echoing Hannah, yes, I have heard that about pinkeye. And pinkeye for members of the family other than the baby.

  6. Platos_Redhaired_Stepchild
    January 9, 2019 at 12:51 am #

    My mother was raised to believe you should train the lefty out of your kid so they’d do better in school. (training, btw, involves tying one hand behind their backs)

    So the peaks and drops in left-handedness are probably due to parents realizing that sh*t is feckin’ crazy.

    • January 9, 2019 at 9:45 am #

      I am strongly left handed and was entirely bottlefed. Also, when my second grade teacher forcibly tried to make me right-handed all that happened was that I developed a bad stutter and couldn’t write at all for two years. My mother, a secretary, taught me to touch type. I was in high school before my handwriting was easy and legible — and using my left hand.

      • sdsures
        January 9, 2019 at 8:56 pm #

        I’m so sorry for what your teacher did. I am also a leftie, but was never made to try and switch.

        • January 10, 2019 at 5:00 am #

          Well, I can now type 80 words a minute…

          • sdsures
            January 10, 2019 at 2:00 pm #

            Do you by chance play any musical instruments? I’ve played piano since I was 5.

  7. Allie
    January 8, 2019 at 11:41 pm #

    He lost me at “those whom dismiss.”

  8. MaineJen
    January 8, 2019 at 4:21 pm #

    Good god. Is this a real article?

  9. Amazed
    January 8, 2019 at 3:25 pm #

    OT: Today, it’s Granny’s Day where I am. At least, that’s the literal translation… Actually, it’s Midwives’ Day. Granny midwives were really respected and revered, rightfully so. The word we use for what granny midwives did is literally “granny”, like “birth” that has become a verb, which is what leads many people to think we celebrate our own grandmothers. We don’t.

    IOW, it’s Delivery Help Day.

    Happy Midwives’ Day to everyone who does what midwives did at the times they were the only ones standing between mother, child, and death! Happy Midwives’ Day, Dr Amy, Addi, Antigonos and all obstetricians, CNMs, doulas, and healthcare workers engaged in mother and child safety!

    • January 9, 2019 at 9:46 am #


      • Amazed
        January 10, 2019 at 6:32 am #

        With your no nonsense attitude, I can easily picture you as one of the granny midwives who shooed men out of house and took the matter into their own hands, always aiming for the best results possible with the knowledge they did have. I think you would have enjoyed to have all mothers of (current – the distinction is very important) toddlers lined up with the children before your door. Not so sure you would have liked the ritual bathing – it IS January, with snow and all…

        • Cristina
          January 11, 2019 at 4:30 pm #

          Polar bear dips are common on January 1 here in Canada. They just cut a huge hole in the ice. Personally, I think they’re nuts.

    • Zuul
      January 9, 2019 at 4:35 pm #

      May I ask where you are from? What a cool day to celebrate!

      • Amazed
        January 10, 2019 at 5:17 am #

        Eastern Europe. Bulgaria. And yes, it’s a very cool day with centuries-old traditions. Mothers of young children bathed them and brought them over to the village midwife to get a blessing, rubbing in honey and butter, bring presents to the midwife, and generally hold rituals meant to safeguard health. They gathered into the midwife’s house and brought things for a feast, held the ewer as the midwife washed her face, and so on. In some regions, she was even escorted to the village well or river for a ritual bathing. It’s a women’s holiday and men were not welcome.

        I can see why current midwives might aspire to this revered status. The difference is, midwives of old did all they could to actually safeguard life. They didn’t scoff at risks.

  10. Angharad
    January 8, 2019 at 2:37 pm #

    Is… is this a thing we’re supposed to care about? In all my parenting worries, I’ve never said a silent prayer that my babies would turn out right-handed. This sounds like even less of a benefit than two extra IQ points.

    • kilda
      January 8, 2019 at 7:50 pm #

      seriously. what a non-benefit! I’m left handed and I’ve had a pretty good life. I mean, left handed scissors don’t work very well, and it was harder learning to knit, but that’s not exactly life-changing.

      • January 9, 2019 at 9:47 am #

        I just mirror-image everything and work “backwards”. I’m a big knitter and crocheter.

      • FormerPhysicist
        January 9, 2019 at 11:47 am #

        I’m left handed, and find crocheting and knitting easy (I do knit continental). I did insist that my left-handed child learn to use right-handed scissors with her right hand. In fact, that was the one goal I stated at the first quarter kindergarten conference.

      • sdsures
        January 9, 2019 at 8:57 pm #

        I’m left handed but my grandmother taught me to knit right handed. I make a pretty good living at it.

  11. BeatriceC
    January 8, 2019 at 2:15 pm #

    Both of my parents (who were both breastfed according to my grandmothers) are left handed. I was left handed until my catholic nun kinder teacher forced me to go right. These days I do most gross motor skills left handed and fine motor skills right handed, though if I can’t use my right hand (happens from time to time as a result of old injuries from a mountain climbing accident) I can function reasonably well, including writing, with my left hand. One of my sons is left handed, and he happens to be the one that EBF’d the longest. Another child was both-handed until a teacher forced him to choose. Up until the second round in first grade he would hold two pencils and work from the left margin to the center with his left hand and then switch to his right from the center to the right margin. Based on his natural instincts with gross motor skills, I’m guessing he’d have eventually settled on his left hand had he been left to his own devices, but when you’re 7 and your teacher tells you to pick a hand, then you look around and see literally every other kid using their right hands, it makes sense that you would also use your right hand. The third kid is strongly right handed. For additional data my ex is right handed, his mother was left handed and his father was right handed. My only other fully biological sibling is right handed (I have siblings adopted from other family members), both of my grandmothers were right handed and both of my grandfathers were left handed. My father is the only leftie in his sibling group and at least two of my mother’s five siblings were forced to switch to right for the same reason I was (why my mother escaped that I don’t know). Of my 11 (known-a few of my uncles probably have kids they didn’t know about), two are lefties, and one is unknown (one of my uncles has a kid we know about, but he basically abandoned her when she was an infant, and since the mom lives in Finland, the family lost contact.).

    So while all this is limited data, it’s definitely strong evidence that maybe, just maybe, handedness is a genetic thing.

    • Gene
      January 8, 2019 at 4:04 pm #

      You weren’t the only nun-forced right hander! According to my parents, I was left handed until my crazy ex-nun first grade teacher whacked me with a ruler every time I held anything with my left hand. I also write “left handed”. I hold a pen in my right hand, but twist my wrist so that I write backwards like a lefty does. It’s super bizarre.

      My sis breastfed her (fraternal) twins for almost 15mo. One is a lefty, the other is a righty. Go figure.

      • demodocus
        January 8, 2019 at 6:30 pm #

        My parents’ non-nun public school teachers forced them to write right handed too.

        I’m mildly surprised that it looks like both my kids are righties, not because of how they were fed but because their parents are both lefties. (At 5, BoyBard is a likely rightie, though not as strongly right as I am left, at 2, GirlBard is still pretty flexible, but she nearly always draws with her right.)

        • Allie
          January 8, 2019 at 11:49 pm #

          Epigenetics : )

          Your parents’ trauma suppressed the lefty gene.

        • guest
          January 9, 2019 at 11:19 am #

          Both of mine are lefties and we have no one in the family that we can think of on either side that is a lefty. It’s always fascinating to the pediatrician.
          We just Google when needed to help them figure out how to navigate as lefties (i.e. how to hold a pencil for writing).

  12. Megan
    January 8, 2019 at 12:57 pm #

    The most important question about this study’s result is, “Who cares?”

    • KQ Not Signed IN
      January 8, 2019 at 1:09 pm #

      Exactly. How is it a benefit? Unless you believe lefties are inferior – as some people actually do. Which is beyond bonkers.

      Also, my husband and I are both right handed and my breastfed only son is a lefty. So OBVIOUSLY this proves…I don’t know, I don’t care enough to even come up with a punchline.

      • LaMont
        January 8, 2019 at 2:49 pm #

        I mean, in a world where a looming theocratic autocracy is upon us, any little bit of “normal” helps?

      • Allie
        January 8, 2019 at 11:53 pm #

        Not inferior… EVIL (Duh, duh, DUH!)! The Latin word for left is literally “sinister.”

        • KQ Not Signed IN
          January 9, 2019 at 11:53 am #


          Actually, that would explain a lot about my kid.

          Carry on.

    • AirPlant
      January 8, 2019 at 2:03 pm #

      Well, you see as a person who heavily subscribes to superstitions theories from the dark ages it is deeply important to me to impact the handedness of my future children so they may never come under suspicion of witchcraft.

    • Young CC Prof
      January 8, 2019 at 8:45 pm #

      There is a detectable relationship between handedness and rates of accidental injury, due to stuff not being built for lefties, but yeah, it’s a pretty silly thing to worry about, and a totally absurd way to actually address the problem.

      • swbarnes2
        January 8, 2019 at 11:11 pm #

        Apparently, six of the last twelve presidents were left handed. These breatfeeding mom’s are killing their kids chances of being in the oval office!

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