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.”