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Mums, Babies, and Their Brains: Why They Take Sides

A new study shows lateralization influences patterns of social interactions

Many years ago, Dr. Lee Salk observed that mothers preferred to cradle their babies on their left side, suggesting that mom's heart beat was soothing to infants. This is called the "heartbeat hypothesis." Over time, his hypothesis fell out of favor, and other researchers suggested that when a baby heard their mother's voice in their left ear, the right side of their brain was involved in processing the sounds. And, because the right side of an infant's brain is more developed than the left, the baby can better process their mother's sound. This is called the "mom's voice" hypothesis.

Pixabay free images
Source: Pixabay free images

Now, a recent study by Karina Karenina and her colleagues published in Nature with the title "Lateralization of mother–infant interactions in a diverse range of mammal species," shows that mothers and infants position themselves so that the right side of their brain is involved in processing social information. The abstract for their study, which is available online, reads as follows:

Left-cradling bias is a distinctive feature of maternal behaviour in humans and great apes, but its evolutionary origin remains unknown. In 11 species of marine and terrestrial mammal, we demonstrate consistent patterns of lateralization in mother–infant interactions, indicating right hemisphere dominance for social processing. In providing clear evidence that lateralized positioning is beneficial in mother–infant interactions, our results illustrate a significant impact of lateralization on individual fitness.

Brain lateralization refers to structural and functional differences between the left and right hemispheres of the brain.

The title of the original paper didn't catch my eye as did the title of a succinct summary by Alice Klein called "Why mums and babies prefer to keep to one side of each other." Also available online, we learn that when these researchers studied "horses, reindeer, antelopes, oxen, sheep, walruses, three species of whale and two species of kangaroo ... Infants of all species were more likely to position themselves so that their mother was on their left. This happened about three-quarters of the time."

However, there's more to the story. When a threat is present, the roles reverse and "'Infants keep their mother on their left in normal situations such as moving forward or suckling ... But when faced with stressful situations such as when fleeing, mothers prefer their infant on their left side so they can better monitor them.'”

The evolution of lateralization

All in all, mothers and infants consistently try to use their right hemisphere to process social information. So, is there some evolutionary advantage to this lateralization pattern? Renowned neuroscientist Dr. Lesley Rogers, co-author of the fascinating book called Divided Brains: The Biology and Behaviour of Brain Asymmetries, notes, “'If you’ve got different functions to perform, you can do that more effectively if you allocate different kinds of processing to each brain hemisphere ... So it makes sense for the right hemisphere to be dedicated to social behaviour'” (for further discussion, please see "Divided Brains: Fascinating Facts about Brain Asymmetries").

The final chapter of Divided Brains makes it clear why knowledge about brain lateralization is important to those interested in situations that animals find pleasurable or stressful and how various stimuli affect their well being. For example, horses are more likely to attack conspecifics seen in their left visual field (inputs from the left eye are processed by the right hemisphere) and they show stronger fear responses to humans or novel stimuli seen with their left eye than with their right eye. Horses also use their right nostril to sniff unfamiliar stallions. Dogs, too, react more fearfully to stimuli seen on the left and processed in the right hemisphere, and they use the right hemisphere to process disturbing sounds, such as thunder and barks of other dogs that are arousing and distressing. Dogs also use their right nostril to sniff arousing odors, which also means use of the right hemisphere since inputs from each nostril are processed by the hemisphere on the same side as the nostril (i.e. no crossing the midline as for vision).

It's pretty clear that brain asymmetries have evolved across many different species and much remains to be discovered about how they influence patterns of social interaction. The study by Karina Karenina and her colleagues provides a fascinating glimpse into the ways in which brains evolve and the role they play in influencing patterns of social encounters. I look forward to further comparative studies on a wider range of species focusing on the relationship between lateralization and social behavior.


Karenina, K., Giljov, A., Ingram, J., Rowntree, V. J. & Malashichev, Y. Lateralization of mother–infant interactions in a diverse range of mammal species. Nat. Ecol. Evol. 1, 0030 (2017).