Why do most mothers cradle their baby on the left? Researchers have found a profound reason
During the first 12 weeks of a newborn's life, 80pc of mothers tend to hold the baby on the left. After dismissing theories about hand dominance, researchers now think it could be something more profound. Shane Cochrane investigates
Cradling a baby is such a simple and instinctual act that most of us have never stopped to think about how or why we do it. So you might not have noticed that most mothers - about 80pc of them - cradle their baby on the left side of their body.
You'll find this "left-cradling bias" across all countries and cultures.
You'll also find it in the animal kingdom.
In 1989, for example, researchers from the University of Liverpool found that, much like humans, chimpanzees, gorillas and orangutans showed a significant preference - an average of 80pc - for cradling their infants on the left side of their bodies.
Scientists have taken a real interest in this cradling preference in recent years. Given that it may be the product of millions of years of evolution, some of those scientists are interested in what it can tell us about how our brains evolved.
Others, however, are curious about what it can tell us about the here and now - and possibly the future.
Since it was first recognised, there have been a number of attempts to explain our left-cradling bias.
For a long time it was linked to hand dominance. A right-handed mother, according to this thinking, cradled her baby on the left to keep her dominant hand free. However, a study by the American psychologist Lee Salk revealed that 78pc of left-handed women also cradle their babies on the left.
Salk's own theory was that left-side cradling kept the baby close to its mother's heart, the beating of which is incredibly soothing infants. But even though Salk demonstrated that listening to a recording of a heartbeat made for happier and healthier babies, his theory never really took hold.
Today, the prevailing theory is based around how our brains are structured, in that mothers cradle to the left because it keeps baby in their left visual field, which is linked directly to the brain's right hemisphere. And this is the dominant hemisphere when it comes to identifying faces and their emotional expressions.
"We call these hemispheric dominances cerebral lateralisation, and it may date back about 500 million years," explains Dr Gillian Forrester, of Birbeck, University of London. "What we think is happening is that if you develop a brain that has dominances on either side, rather than replicating everything across both hemispheres, it's not only a more efficient use of the neural space but it also means that you don't have both hemispheres competing and giving conflicting responses.
"And it allows the organism to do two things at once. One hemisphere is really good for looking out for threats in the environment, and the other hemisphere is very good at doing repetitive sorts of motor sequencing. And this allows an organism to watch for predators and feed itself at the same time.
"And because of the way the motor system is wired up, the left hemisphere controls the right side of the body and the right hemisphere controls the left side of the body. So, if you're doing something biased to one side of your body, it means that it's the opposite hemisphere that's dominantly controlling that behaviour.
"And we know with the identification of emotions it seems to be the left visual field, and therefore the right hemisphere, which is dominant in most people."
In the case of left-cradling bias, by having her baby in her left visual field a mother can quickly identify and react to its emotional state.
Cradling in this way also presents the left side of the mother's face to the baby. The left side of the face displays emotions earlier and more intensely than the right side. And scientists believe that having the more expressive side of the mother's face in the infant's left visual field may help with bonding and social development.
Mothers don't always cradle their baby on the left, of course. It's not always practical - or comfortable. And some researchers believe that the left-cradling bias is only dominant in the first 12 weeks, when the infant is most vulnerable.
But others believe that we can learn a lot by studying how mothers - and others - cradle an infant.
Dr Nadja Reissland of Durham University found that a mother unconsciously changed cradling sides according to how she was interacting with her baby: cradling to the left if her baby needed soothing; and to the right if she wanted to gain the baby's attention.
Previous studies have established that mothers use their voices to do this - speaking in a low pitch when calming a distressed baby and a higher pitch when encouraging attention, but by monitoring mothers in their homes, Reissland found that many would intuitively combine cradling on the left and speaking in a low pitch to soothe their baby, and cradling on the right and speaking in a high-pitched voice to gain their attention.
In a later study, Dr Reissland found that cradling preference might also be an unconscious expression of the mother's emotions.
In this study, in addition to monitoring the mothers and infants at home, she had each mother complete a survey to establish their mental state. Through this survey, Reissland discovered that many of the mothers were experiencing extreme stress, and that 32pc of those mothers cradled their babies predominantly on the right.
"Many mothers don't realise they are suffering from stress, or don't want to admit they are," said Dr Reissland on publishing her results. "The way they interact with their child is usually the best indicator of their mental state."
More recently, in a University of London study led by Dr Gillian Forrester and Dr Brenda Todd, 96 children - aged five to six - from a primary school in London took part in a number of cradling trials.
Each trial involved a child being asked to cradle either a pillow; a "proto-face" pillow, which is a pillow that has a face on it made up of three dots and a circle; a human doll; and an orangutan doll.
As well as observing how they interacted with the dolls, the researchers assessed each child's social and communication abilities.
Forrester and Todd found that, overall, the children held the human doll significantly more often in the left cradling position. They cradled the pillow with the face drawn on it in the same way. The orangutan doll was predominantly cradled on the right. And the children showed no preference, left or right, for cradling the regular pillow.
When the researchers compared the cradling results to the social and communication ability scores, they found that those children who had a greater tendency to cradle the human doll and the "proto-face" pillow to the left scored significantly higher than the children who cradled them to the right.
So, what does this mean?
Well, Forrester and Todd believe that their research may help identify young children with neurodevelopmental problems.
"We're looking at population patterns here, so you wouldn't use these results to suggest a specific child has an issue," says Dr Forrester. "But we do know that children with some neurodevelopmental disorders, like autism, tend to have weakened cerebral lateralisation. So it suggests that we could be using tests of behavioural biases like visual field as a risk marker.
"Weak behavioural dominances may be a risk marker for later cognitive development. And it's something that actually could be a very useful indicator for further testing - because current diagnostic tests for autism rely a lot on cognitive outcomes, like how well does the child speak or express themselves socially?
"And you have to wait quite a long time to test those things because children don't typically develop those skills until they're three and four-years-old.
"And therefore you're getting these diagnoses quite late in the day, which means that interventions also come quite late in the day. But if we think that behavioural biases represent the underlying architecture of the brain when the child is born, then we could be having very early and informative indicators of what we think development will be like for that child." Forrester believes that these early indicators could allow interventions to reinforce good motor skills and stimulate the relevant parts of the brain at the best possible time - when the brain is still "incredibly plastic".
To test their approach, Forrester and Todd are due to run the experiment again - this time with children with autism.
While the University of London researchers were looking for evidence of left-cradling bias in children, researchers at Saint Petersburg State University and the Institute of Experimental Medicine were looking for evidence of the phenomenon in walruses and flying foxes.
What they found was very interesting.
While they discovered that, when resting face-to-face, both walruses and flying foxes tended to keep their infants in their left visual field, they also found that this positioning preference wasn't just the mother's choice.
When walrus pups float alongside their mother, they tend to keep her in their left visual field. And when hanging side-by-side, infant flying foxes also prefer to keep their mother on the left.
"In humans, mothers regulate the physical position of an infant when cradling," reported the researchers. "However, our results show that infants actively choosing side position relative to the mother also keep her predominantly in their left visual field."
Which suggests that, if they were free to choose, human babies would still want to be cradled on the left.
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