For several decades, scientists have wondered why around 80% of women, regardless of whether they are right or left-handed, use their left arms to cradle their offspring , in what appears to be instinctive behavior. The first to suggest an explanation was the American psychologist Lee Salk. Inspired by the behavior of the Rhesus macaque monkeys, native to Asia, he proposed that the maternal heartbeat was the cause of such preferences. Although his theory was extended for a time, later studies showed that, for both the fetus and the newborn, the favorite sound was the voice of its mother, and that heart sounds did not seem to have much importance.
The most reliable hypothesis came a few years ago from Victoria Bourne and Brenda Todd, from the British University of Sussex. These researchers attribute the custom to a sophisticated process in which the cerebral hemispheres of mother and child are involved. Thus, when a child is cradled on the mother's left side, his reactions – crying, laughing, yawning … – go directly to the maternal right cerebral hemisphere , which is where emotional gestures and facial expressions are recognized. Consequently, the relationship is strengthened because the mother has faster and more intuitive access to the child's needs.
