Huge study finds a bizarre link between bottle feeding and left-handedness

Humans

The way infants are fed and nurtured in the first year of life could have something bizarre to do with being left- or right-handed, a new study suggests.

The meta-analysis, which look at over 60,000 mother-child relationships, has now shown that left-handedness is lower among breastfed infants compared to bottle-fed infants.

To be very clear, this isn’t to say that breastfeeding will cause a child to be right-handed, or that bottle-feeding will lead to left-handedness (or that either is better than the other).

After all, research has shown that a person’s dominant hand is determined at least partially by genetics long before birth. And, of course, correlation doesn’t equal causation. 

Instead, the authors are suggesting that breastfeeding could play a fascinating role in which side of the brain controls our dominant hand, a process called cerebral lateralisation.

“We think breastfeeding optimises the process the brain undergoes when solidifying handedness,” explains lead author Philippe Hujoel, an epidemiologist at the University of Washington.

Bringing together data from seven national surveys in five different countries, the researchers noticed that with each month of breastfeeding, there was a downward trend in the number of left-handed infants, and this trend continued until nine months of age, after which it plateaued.

After accounting for other factors that can lead to left-handedness, the authors conclude that this span of time must be a critical window in establishing a child’s dominant hand – a time when brain lateralisation is especially susceptible to the way a child is nurtured.

Among all the bottle-fed infants, the researchers calculated that about one in five cases of left-handedness could be influenced by the fact they were breastfed for less than six months.

The idea is bolstered by the fact that several suspected links to left-handedness are, at least in part, also associated with breastfeeding.

For instance, the researchers found that twins are more likely to be left-handed, maybe because they have to share their mother’s milk. What’s more, premature babies, who are often bottle-fed in hospital, were also found to have higher rates of left-handedness.

“Possibly, the effect of breastfeeding on handedness is unrelated to nutrition, and instead mediated by the hormonal responses associated with mother-infant bonding during breastfeeding,” the authors suggest.

“An alternate interpretation of course is that another powerful hidden confounder, associated with breastfeeding, was missed.”

Of course, whether a baby ends up left- or right-handed – and both are great – the most important thing is for them to be fed, regardless of the method. But the link is interesting to explore further for what it can teach us about handedness and brain development in the first year of life.

This study has been published in Laterality: Asymmetries of Body, Brain and Cognition.

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