Have you ever wondered why most people carry babies on their left side? This question has baffled scientists for years. There are a few tentative answers: we carry on the left side because we hold the baby with our stronger arm, typically the right one. Or perhaps, just the opposite. We are stronger on the left side because that’s where we usually hold our overstuffed purses! Maybe some women carry their babies on their left side to leave their right hand free to do other things. However, a new study conducted by scientists from Saint Petersburg State University in Russia found that this phenomenon, called left-side bias or left-cradling bias, is an expression of the right side of the brain to process emotions and take in information from the world around us. The position of an infant on the mother’s left side may optimize maternal monitoring, by directing sensory information predominantly to the mother’s right hemisphere. What does this mean in plain English?
You might remember from school science that the right hemisphere of the brain controls the left side of the body and vice versa. By having babies on the left-hand side, we process things like emotion, crying, and facial recognition with the right side of our brains. This contributes to bonding because the right side of the brain also aids in making sense of what we see.
Previous research shows that 70-85% of women cradle babies on their left side. In this new study, scientists looked at 11 different species of animals (including walruses, reindeer, orca whales, and kangaroos, as well as humans) and found that in all instances, mothers kept their offspring to the left. The team concluded that this specific positioning allowed mothers to take in information that’s analyzed in the right side of the brain.
This “lateralization” effect may also be evolutionary. For thousands of years, parents of young babies have been bending, lifting, carrying, pushing, dragging, cuddling, and soothing their little ones. For a right-handed person, holding the baby on the left arm allows them to use their right hand freely. It is not a matter of strength or of neurobiology. It is an automatically ingrained choice. After all, isn’t that why you hold that cellphone with your left hand?