Bottle-Fed Infants Are More Likely to Be Left-Handed Than Breastfed Babies, New Study Finds

"We think breastfeeding optimizes the process the brain undergoes when solidifying handedness," said University of Washington study author Philippe Hujoel

Baby boy drinking milk at feeding bottle
Photo: Miguel Sanz/Getty

New research suggests that people are more likely to become left-handed or ambidextrous in adolescence and adulthood if they are fed from a bottle as an infant.

The findings, extracted from a study conducted at the University of Washington that was published last month in the scientific journal Laterality: Asymmetries of Body, Brain and Cognition, were based on a group of 60,000 pairs of mothers and children who breastfed or bottle-fed their babies.

Specifically, the study found that babies who breastfed for one month had a 9 percent lower chance of being non-right-handed, while one to six months showed a decrease of 15 percent and more than six months had a 22 percent lower chance of “nonrighthandedness.”

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According to Science Daily, the author of the study — Philippe Hujoel, an adjunct professor of epidemiology at UW’s School of Public Health and professor at its School of Dentistry — said, “We think breastfeeding optimizes the process the brain undergoes when solidifying handedness.”

The conclusion also shed light on how long a mother would need to breastfeed before it had any kind of lasting impact on the brain’s development in terms of whether a child would be right-handed or not — with over six months, as mentioned previously, having the largest impact.

“It provides an independent line of evidence that breastfeeding may need to last six to nine months,” Hujoel explained of the findings, according to Science Daily. (However, “Breastfeeding infants after nine months was no longer associated with further reductions in the prevalence of nonrighthandedness,” the study concluded in part.)

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Hujoel clarified, according to Science Daily, that the method of feeding only suggests an in-part link between itself and handedness, while much of the latter is determined early in the fetal stage.

Instead, feeding may influence handedness during brain lateralization — a later process Science Daily describes as “when the region of the brain that controls handedness localizes to one side of the brain.”

Thus, “It is concluded that the critical age window for establishing hemispheric dominance in handedness includes the first nine months of infancy and is in part determined by nurture,” the study said.

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