Whether you bottle feed or breastfeed your baby could play a role in what hand they prefer to use later in life, according to a new study from the University of Washington. In a review of more than 60,000 mothers and their babies, the researchers determined that bottle feeding infants was associated with a higher prevalence of them being left-handed even when accounting for known risk factors that may influence which hand a child favors.

To determine how feeding an infant may influence their dominant hand, researchers gathered data on the duration of breastfeeding as reported by the mother through seven national surveys in five countries. They found that breastfeeding for more than six months was associated with 3 percent less left-handed babies than those that were bottle-fed, with about one in five left-handed bottle-fed babies possibly being attributable to a lack of breastfeeding for more than six months.

"We think breastfeeding optimizes the process the brain undergoes when solidifying handedness," said study author Philippe Hujoel in a statement. "That's important because it provides an independent line of evidence that breastfeeding may need to last six to nine months."

Writing in the journal Laterality: Asymmetries of Body, Brain and Cognition, the scientists say their study sheds light on when areas of the brain, such as those that control handedness, localize to one side in a process known as brain lateralization. Breastfeeding, they write, may optimize this lateralization toward becoming right- or left-handed during a critical window of time during the first nine months of infancy.

“[Our] current findings thus suggest that the critical period of cerebral lateralization, the time window wherein brain lateralization is susceptible to nurture, starts sometime before the 3rd month in utero. The developmental origins of laterality and ends in infancy before the age of 9 months,” said Hujoel.

According to the paper, previous studies have found that “breastfed infants have been reported to have increased righthandedness, increased intelligence, increased head circumference, decreased speech problems, and decreased multiple sclerosis,” while other studies have interpreted these associations as causal based on evidence that breastfeeding leads to more grey matter volume in the brain. However, these studies don’t always accurately account for socio-economic factors and health awareness (among other factors) due to the difficulty in parsing out confounding factors.

The study does not imply that breastfeeding alone dictates what hand a child will favor later in life, as this is determined during fetal life and at least partially by genetics. Further, it’s important to note that although they used a random sample to reduce selection bias, used trained examiners to reduce information bias, and that public availability of the data allowed for independent verification of reporting biases, the researchers note that further, broader research is needed.