Breastfeeding May Determine Whether Your Baby Will Be Right- or Left-Handed

While genetics still play a role, researchers found that bottle-fed infants were more likely to be left-handed than their breastfed counterparts.

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Photo: wong sze yuen/Shutterstock

January 11, 2019

A new study from the University of Washington gives some insight into one of parenthood's most interesting mysteries: what determines whether our kids become right-handed or left-handed? Is it nature, nurture, or some combination of both?

"Fetal handedness is at least in part determined genetically; evidence of this genetic determination of handedness may start as early as the start of the second trimester of pregnancy," says study author Phillippe Hujoel, Ph.D.

According to Hujoel, the most popular current theory about the relationship between genetics and handedness is that some babies may carry genes which determine right-handedness; any baby who doesn't carry these genes would then have an equal chance of becoming either right- or left-handed.

Okay, so nature definitely plays a part...but what about nurture?

Dr. Hujoel points out that experts already know how something called "brain lateralization" (i.e. the left-brain vs. right-brain division) influences handedness—at some point in early brain development, one side of our brain takes over the control of handedness, turning us into either lefties or righties. Hujoel's study suggests that breastfeeding may contribute to or "optimize" that brain process.

In the study, Dr. Hujoel compared more than 60,000 mother-baby pairs to look for a connection between handedness and feeding method (either breastfeeding or bottle-feeding). He found that babies who were breastfed for six to nine months were less likely to be left-handed, meaning that bottle-fed infants were more likely to be left-handed than their breastfed counterparts.

Now, researchers can't make the claim that breastfeeding directly causes right-handedness—again, genetics definitely play a role. But the study would seem to confirm that nurture also plays a significant role.

"Many studies have provided equally strong evidence that nurture (or environment) interacts with genetic factors," says Dr. Hujoel. "Short gestation, for instance, increases the likelihood for left-handedness. Possibly, breastfeeding enhances the ongoing lateralization towards either right- or left-handedness."

As for why, exactly, breastfeeding might be so influential in this area, Dr. Hujoel says there are several possible explanations, including the unique nutritional value of breastmilk and the hormonal responses to the mother-infant bonding experience (or something else entirely that researchers haven't figured out yet).

The study also underscores the potential benefits of breastfeeding for a minimum of six to nine months. The findings show that when babies were breastfed for three periods of time (less than one month, one to six months, and more than six months), the prevalence of left-handedness decreased with each duration (from nine percent to 15 and 22 percent, respectively). In other words, breastfeeding had the most impact on brain development between six and nine months.

However, there is a limit: beyond nine months, the study shows no significant decrease in left-handedness.

Obviously, whether your baby turns out to be a rightie or a leftie is no big deal (your left-handed kiddo could turn out to be a great artist or southpaw pitcher one day!). But the study does further highlight the fascinating connection between breastfeeding mothers and their babies' brains.

"A large body of research has evaluated the associations between breastfeeding and overall infant brain development and function," says Dr. Hujoel. "[This] study adds a tiny piece of evidence to this larger database, specifically focusing on how breastfeeding is associated with brain lateralization, which is just one aspect of overall infant brain development."

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