A new study of about 60,000 mother-infant pairs finds that left-handedness in babies is less common among those who were breastfed as compared to those who were bottlefed.

The findings, published in the journal Laterality: Asymmetries of Body, Brain and Cognition, provide further insight into the development of complex brain functions which ultimately determine which hand the infant will likely use.

Controversy remains over whether breastfed and bottlefed infants have distinct neurodevelopmental life courses. Some research has shown that breastfed infants show increased right-handedness, increased intelligence, increased head circumference, decreased speech problems, and decreased multiple sclerosis.

These associations have been interpreted by some as causal and explained on the biological basis that breastfeeding leads to increased myelinization and grey matter volume. However, others say these are assumptions based on a faulty line of reasoning — a reflection of the inability to disentangle breastfeeding from confounders such as socioeconomic factors and health awareness.

Therefore, the goal of the new study was to initiate a systematic search for existing data sources to explore the association between breastfeeding and non-right-handedness.

“We think breastfeeding optimizes the process the brain undergoes when solidifying handedness,” said Dr. Philippe Hujoel, the study’s author, a professor at the University of Washington’s School of Dentistry and an adjunct professor of epidemiology at the School of Public Health.

“That’s important because it provides an independent line of evidence that breastfeeding may need to last six to nine months.”

Importantly, the study does not imply, however, that breastfeeding leads to right-handedness, Hujoel said. Handedness, whether it be right- or left-handed, is set early in fetal life and is at least partially determined by genetics.

Instead, the study sheds light on when the region of the brain that controls handedness localizes to one side of the brain, a process known as brain lateralization. Possibly, the research shows, breastfeeding optimizes this lateralization towards becoming right- or left-handed.

For the study, the researchers analyzed data of seven national surveys in five countries involving 62,129 mother-child pairs. These surveys had low risk of bias.

A meta-analysis showed that breastfeeding for < 1 month, 1 to 6 months, and > 6 months, when compared to bottle feeding, was associated with a 9 percent, 15 percent and 22 percent decreased prevalence of non-right-handedness, respectively.

Breastfeeding for longer than 9 months was not linked to further reductions in the prevalence for non-right-handedness. It is concluded that the critical age window for establishing hemispheric dominance in handedness includes the first 9 months of infancy and is in part determined by nurture.

Source: University of Washington