For decades, scientists have shown interest by a fact verified by studies: that 80% of mothers, are right-handed or left-handed, cradling their offspring with the left arm. So far, it was considered that it was a distinctive feature of humans and great apes, though its origin and evolutionary explanation remain a mystery.
Cradling the baby in the left arm enhances the mother-child relationship, the reactions of the baby are processed by the right cerebral hemisphere of mother, commissioned to recognize facial expressions
For a long time it was considered valid thesis, defended by the American psychologist Lee Salk, that this was due to the location of the maternal heart, whose heartbeat could help calm babies. However, subsequent investigations found that the mother’s voice was much more important in this regard that the sound of the heart.
Another theory, known as lateralization, which involves the cerebral hemispheres in the cause of this predilection has gained strength in recent years. Thus, according to recent findings, cradling the baby in the left arm would improve the mother-child relationship, since the reactions of the baby are processed directly by the right cerebral hemisphere of mother, responsible for social processing and, therefore, to recognize children’s facial expressions (crying, laughter, yawns, the stews…). In this way, the mother reacts before the physiological needs of the baby.
The lateralization left, a feature common in mammals
A recent study, carried out by researchers from the universities of Petersburgo (Russia), Utah (USA) and Tasmania (Australia), and published in the journal Nature, Ecology and Evolution, has wanted to verify if really this effect of lateralization towards the left was human and primates or, on the contrary, was also common to different mammalian species.
Research the Kangaroo, the horse, the killer whale, or the beluga discussed ten species of mammals, among them. In the analysis, scientists found a bias to important population level, whereby the offspring showed preference for keep mother on the left side, regardless of the species or type of behaviour (with the exception of reindeer in the rest). In this sense, the authors of the study that the bias of the left side remained in babies, not only in routine behaviour, but also in stressful situations, such as when fleeing.
This fact demonstrates, according to the researchers, that humans are one of the many species that show a strongly lateralized trend in spatial relations mother and infant, which would probably indicate that this feature has an ancient evolutionary origin in mammals. The findings also suggest that the sensory lateralisation facilitates the bond between mother and son.