Breastfeeding May Influence Babies’ Handedness

About 90% of people are right-handed and about 10% are left-handed, but why some of us become left-handers and others do not is still largely a scientific mystery. A new study published in the scientific journal Laterality this week might shed some light on this mystery.

Lead by Prof. Philippe P. Hujoel from the School of Dentistry at the University of Washington in Seattle, the study investigated if breastfeeding affects whether the baby will be left- or right-handed later in life (Hujoel, 2018). Prof. Hujoel integrated data from seven large survey studies from five different countries and found a surprising effect: In the more than 60,000 mother-child pairs, he discovered that there was a clear association between breastfeeding and handedness!

The scientist classified children into right-handed and non-right-handed (left-handed and ambidextrous) and found that breastfeeding clearly reduced the chance of the child being left-handed or ambidextrous. Compared to bottle feeding, breastfeeding for less than one month decreased the chance of the baby later being left-handed or ambidextrous by 9%. Breastfeeding for a duration of one to six months decreased the chance of the baby later being left-handed or ambidextrous by 15%. The strongest effect was found when mothers breastfed for more than six months. In this group, the chance of the baby later being left-handed or ambidextrous decreased by as much as 22%. This meant that breastfeeding for more than six months decreased the absolute prevalence of left-handedness or ambidexterity from 13% to 10%—which means that about every fifth case of non-right-handedness in the bottle-fed group might be due to the lack of breast-feeding. Breastfeeding for more than nine months did not further affect handedness than breast-feeding for six months.

Why might breastfeeding affect handedness, you might ask? First off, it has been shown that left-handedness has some genetic influences, but is largely determined by environmental factors. A large-scale twin study that analyzed handedness in twins, their parents and siblings in more than 25,000 Australian and Dutch families found that only about 25% of the individual variance in handedness can be explained by genes, while 75% are determined by environmental influences (Medland et al., 2009). Handedness researchers have long suspected that many of these environmental factors influence handedness very early in life, in the period before birth or shortly after, as for most of us it is determined pretty early in life whether we are a “leftie” or a “rightie.” Said environmental factors include, for example, the season of birth, stress during birth, and early visual experience of the hands. Breastfeeding has been suggested to be one of these factors (Denny, 2012), and while it is not possible to infer any strong causal mechanisms from epidemiological data like that presented in the work by Prof. Hujoel, there are two likely explanations why breastfeeding affects handedness.

On the one hand, breastfeeding has been shown to alter early brain development, with increased white matter development in frontal and associated brain regions (Deoni et al., 2013). Moreover, breastfed children have been shown to have better cognitive functions and higher IQs later in life (Horta et al., 2015). As handedness also originates in the brain, it is conceivable that breastfeeding affects the development of handedness-relevant brain regions such as the motor cortex, and indeed this is suggested by recent findings in rhesus macaques (Liu et al., 2019).

On the other hand, Prof. Hujoel suggests that the hormonal responses associated with bonding between mother and baby during breastfeeding might also affect handedness. While there are presently no studies suggesting that oxytocin, the human bonding hormone, directly affects handedness, it has been suggested that sex hormones such as progesterone or testosterone can affect functional hemispheric asymmetries in the brain (Geschwind & Galaburda, 1985; Hausmann, 2017). Since handedness is one form of such left-right-differences in the brain, it is indeed conceivable that hormonal responses might link breastfeeding and handedness, but more research is of course needed to clarify the causality here.