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.
Published last month in the journal “Laterality: Asymmetries of Body, Brain and Cognition,” the study specifically investigated links between how long a mother breastfeeds and the prevalence of non-righthandedness in their kids. The category “non-righthanded” includes the left-handed and the ambidextrous.
Only one in ten people is left-handed, University College London researcher Chris McManus wrote in a book chapter about the phenomenon: “Broadly speaking, the vast majority of humans seem to have been right-handed since the emergence of the genus Homo, some three to four million years ago.”
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The University of Washington researchers found that non-righthandedness was 9 percent less common in kids who were breastfed for less than a month. In kids breastfed for one to six months, non-righthandedness was 15 percent less prevalent, and non-righthandedness was 22 percent less prevalent in kids breastfed for more than six months.
But breastfeeding for more than nine months “was not associated with further reductions in the prevalence for non-righthandedness,” researchers wrote in the abstract of the study.
Researchers said the study “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.”
Study authors cautioned in a university news release that the study doesn’t imply a cause-and-effect link between breastfeeding and right-handedness. Researchers also said handedness is at least in part genetic.
But the study does illuminate how the brain develops in early childhood to determine which hand a child uses more dominantly, according to researchers.
“We think breastfeeding optimizes the process the brain undergoes when solidifying handedness,” study author Philippe Hujoel, a University of Washington professor in dentistry and public health, said in a statement. “That’s important because it provides an independent line of evidence that breastfeeding may need to last six to nine months.”
There’s an ongoing discussion — and in some corners, controversy — over whether new parents should breastfeed or bottle feed infants. A British hospital has even been accused of shaming women who choose to use formula by referring to the practice as “artificial feeding,” the New York Post reported last year.
“Mommy shaming is real!” reality TV star Khloe Kardashian wrote in a Tweet last year. “But the truth is I’ve tried and tried and tried to breast feed only and it wasn’t working for me.”
Families are left weighing the convenience of formula against the reported health benefits of breastfeeding, which has been linked to increased intelligence and head circumference, as well as decreases in speech problems and multiple sclerosis, according to the study authors.
Researchers said it’s possible the link they discovered between breastfeeding and handedness has more to do with the maternal bonding it fosters than with nutrition — or that some other hidden, breastfeeding-related factor might be behind its association with handedness.
“The safest conclusion may therefore be that handedness is related to nurture,” researchers wrote.