If you find yourself a little unbalanced, like maybe your left arm is noticeably stronger than your right, you might blame your overstuffed purse. Or you might give credit to the babies in your life, because whether or not you're a mom, humans seem to favor holding tiny tots on the left side. Scientists didn't know why — until now. In a study published in Nature Ecology and Evolution, the phenomenon, which is called left-side bias or left-cradling bias, encourages the right side of the brain to process emotions and take in information from the world around us. "Left-cradling bias is a distinctive feature of maternal behaviour in humans and great apes, but its evolutionary origin remains unknown," the authors wrote. Previous research shows that 70-85% of women cradle babies on their left side. In this new study, scientists looked at 10 different species of animals (including walruses, 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. You might remember this fun fact from science class: The left hemisphere of the brain controls the right 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. It contributes to bonding because the right side of the brain also aids in deciphering and dissecting what we see. "Our findings suggest that sensory lateralisation facilitates mother–infant bonding," the study reads. Next time you feel like shifting an infant to your right side, you might want to think again. A tired arm might just be worth it, since you'll be making memories and building strong bonds with that little one.