Why do women prefer to cradle their babies on their left side?
Posted October 15, 2018
Over the years, psychologists have explored an intriguing parenting behavior. When mothers hold their babies, they are much more likely to cradle young infants on the left side than on the right.
When you ask women why they display this preference, right-handed women typically say that they hold their babies in the left arm so that they can keep their right hand free for other tasks. Well, that makes sense! But, if you ask left-handed women the same question they will tell you, “It’s because my left arm is stronger.”
Both left- and right-handed women show a bias of holding young babies on the left, especially when they are just enjoying their baby’s company and are not engaged in other activities. For a long time, researchers have been looking more deeply into this preference, and a number of possible explanations have been proposed and tested.
Lee Salk, a psychologist from New York University, initiated the interest in this question in the 1960s. In his regular walks past the monkey enclosure in Central Park Zoo, he noticed that a mother rhesus monkey always held her infant on her left side, close to her heart. He became so intrigued that he researched the behavior of human mothers at a nearby maternity hospital. Here, Salk observed that approximately 70 to 85 percent of the mothers also cradled their babies on the left, regardless of their reported hand preference.
Salk speculated that the colloquial expression “close to a mother’s heart” might signify a real psychobiological process. He believed that a left position was adopted because it maximized exposure to the soothing sound of the mother’s heartbeat, which the fetus had experienced prior to birth—regardless of whether they were humans or monkeys! It is a delightful proposition but not one supported by recent research. The bias to hold on the left is still apparent when babies are positioned where heart sounds are not accessible, for example, when their head is on mother’s shoulder or in the crook of her arm.
More complex explanations of the left cradling bias have been considered in a recent paper. One relates to a very young infant’s own tendency to rest their head to the right, rather than left side; about two-thirds of babies show this inclination. When held on the left, such babies will be most comfortably placed—they can rest their head on the mother’s body and have the best visual access to her face. Well, that theory also makes sense. However, a rigorous test matching the babies’ head placement to the mother’s holding side when she picks the baby up as well as her habitual holding side preference showed no significant correspondence between the two measures.
A recent paper considers a more complex explanation. Researchers now favor the proposal that a mother holds her baby on the left side in order to position the infant within her left visual field. This unconscious placement may reflect an evolutionarily old mechanism by which information from the infant, such as their physical and emotional state, is transferred to the opposite hemisphere, the right hemisphere, of the mother’s brain. In most people, the right hemisphere specializes in perceiving social signals and processing emotional information. Left cradling, therefore, maximizes the holder’s alertness to changes, which could indicate potential threats to the infant’s well-being and survival. Placing the infant in the left visual field also allows the mother to make the most of loving communication with her baby because she is more attuned to their current emotional state.
As Salk noticed, it is not just human mothers who need to be vigilant about infant survival; more recently a side bias for positioning offspring has been found in other mammals, including species as diverse as chimpanzees, walruses, and even some bats.
The hemispheric specialization explanation is most applicable when human infants are very young. As they become less vulnerable and gain greater control of their own movements, the left holding bias reduces, typically at about four months of age.
Now, you may be wondering if fathers, like mothers, show a left cradling bias. Research shows that fathers of newborns do; even young men and women who don’t have babies of their own will show a bias to hold a “baby” doll on their left side—a bias which disappears when they hold other objects of a similar size and weight. The preference to hold a doll on the left side appears early in development and is even found in pre-school girls—and in little boys too, if you can persuade them to hold a doll.
There are still lots of open questions about the full function of the left cradling bias. In particular, researchers want to know more about whether cradling position has lasting social as well as survival implications.
So, you might like to imagine you are holding a newborn baby and discover which would be your own preferred positioning. Don’t worry too much if you are one of the 20 to 30 percent of people who naturally adopt a right hold. Not everyone has typical hemispheric organization and your preference may just reflect what is right for you.
Todd, B.K. & Banerjee, R.A. (2018). Lateralisation of infant holding by mothers: a longitudinal evaluation of variations over the first 12 weeks. Laterality: Asymmetries of Brain, Body and Cognition. 21(1). pp. 12- 33.