Abstract
About 66–72% of human mothers cradle their infants on their left side. Given that left-cradling exposes the baby’s face to the mother’s left visual field (i.e., mainly projected to her right hemisphere) and is altered by emotional states such as stress, maternal left-cradling was interpreted as reflecting right-hemispheric dominance for emotional processing. Whether this phenomenon is unique to human evolution is still in debate. In the present study we followed 44 olive baboon (Papio anubis) mothers and their infants in different social groups. We found that a maternal cradling bias exists and is predominantly towards the left in a similar proportion as in humans, but shifts toward a right bias in mothers living in high density groups. The sensitivity of left-cradling to social pressure highlights its potential links with the mother’s stress as reported in humans. Our finding clearly illustrates the phylogenetic continuity between humans and Old-World monkeys concerning this lateralization and its potential links with hemispheric specialization for emotions, inherited from a common ancestor 25–35 million years ago.
Highlights
About 66–72% of human mothers cradle their infants on their left side
As the right hemisphere of the brain is specialized in the perception of emotional facial expressions[16,17,18,19], the left-cradling bias would favour the mother’s monitoring of the emotional state of the infant
17 kept the same cradling-side bias while only one female switched from an ambiguously lateralized to a left-biased cradling, but both cradling-side bias indices (CBI) had the same negative sign, indicating similar left direction of the cradling-side bias
Summary
About 66–72% of human mothers cradle their infants on their left side. Given that left-cradling exposes the baby’s face to the mother’s left visual field (i.e., mainly projected to her right hemisphere) and is altered by emotional states such as stress, maternal left-cradling was interpreted as reflecting right-hemispheric dominance for emotional processing. Maternal depression involves decreased communication within the mother-infant d yad[26] and a dysfunction of the right brain hemisphere affecting emotional perception It may be considered as a factor that alters the left-cradling b ias[27]. Concerning maternal cradling which involves holding the infant close to one’s body by using the arms and h ands[2], humans are not the only primate species showing this lateralized behavior, as great apes and monkeys like macaques and baboons cradle their babies during the first months of life. This behaviour is mainly female-specific and maternal. No study to date has investigated the link between a left-cradling bias and the maternal emotional state in non-human primates
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