Cuddling linked to how the brain processes emotion.

85% of women hold a baby to their left. Credit: © Creatas

Women are more likely to cradle babies on their left-hand side because it activates bonding-related brain regions, research suggests.

Previous studies have shown that up to 85% of women instinctively clasp infants to their left breast - irrespective of age and maternal status. Some think mothers do this to keep the baby's head close to their hearts, naturally soothing the child. Others rationalize the bias in terms of handedness. Left-handed women say they prefer to hold their infant in their stronger left arm. Right-handers argue it keeps their dominant hand free to attend to the child.

But neither argument fully explains the phenomenon, argue Victoria Bourne and Brenda Todd from the University of Sussex. Instead, it may come down to differences in the brain.

The right side of the brain controls the left side of the body and usually helps to process emotions, explains Bourne. So holding the baby on the left-hand side may help to direct the sight of emotionally charged information, such as tears or laughter, to the specialized right hemisphere for processing, she says.

To find out, the team asked 20 right-handed women to cradle a life-like doll. As expected, over half of the ladies held the doll on their left side. The team then found that these same subjects also use the right side or hemisphere of their brain to process the image of a face laden with emotion1.

"It may help the woman monitor the baby for signs of discomfort, pain or hunger," says Bourne. And, she adds, this may boost mother-infant bonding.

Lateral thinking

But not everyone uses the right side of their brain to process emotional faces. Some use the left-hand side, and others use a bit of both.

The team assessed which side of the brain their subjects used to process emotions by asking them to identify which of two images looked happiest ("take the test":movie.html). The pictures were mirror-image faces, each split vertically down the middle, with one half smiling and the other wearing a neutral expression. People who use the right side of the brain to process emotion find the face smiling on the left-hand side the happiest, and vice versa, explains Bourne.

It's a good task for assessing brain laterality, says Kim Bard from the University of Portsmouth, who studies differences in brain hemispheres. Brain scans can also be used, she says, but these can be time-consuming and expensive, and miss out certain subtle differences.

All of the women in the test who cradled the doll on their left-hand side used their right brain to process faces, the team found. Women who preferred to cradle the doll on their right-hand side were just as likely to use their left brain, right brain, or a mixture of both to process faces.

The result may carry a lesson for new mothers. Women with postnatal depression are often advised to hold their baby on the left-hand side, says Bourne, as this increases bonding for most women. But if the mother actually uses the left side of her brain to process faces, this may not help, she says.

References 1 Bourne, V. J. & Todd, B. K. When left means right: an explanation of the left cradling bias in terms of right hemisphere specializations. Developmental Science, 7, 19 - 24, doi:10.1111/j.1467-7687.2004.00318.x (2004). Download references

Authors Helen R. Pilcher View author publications You can also search for this author in PubMed Google Scholar

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