Abstract
In humans, behavioral laterality and hemispheric asymmetries are part of a complex biobehavioral system in which genetic factors have been repeatedly proposed as developmental determinants of both phenomena. However, no model solely based on genetic factors has proven conclusive, pushing towards the inclusion of environmental and epigenetic factors into the system. Moreover, it should be pointed out that epigenetic modulation might also account for why certain genes are expressed differently in parents and offspring. Here, we suggest the existence of a sensitive period in early postnatal development, during which the exposure to postural and motor lateral biases, expressed in interactive sensorimotor coordination with the caregiver, canalizes hemispheric lateralization in the “typical” direction. Despite newborns and infants showing their own inherent asymmetries, the canalizing effect of the interactive context owes most to adult caregivers (usually the mother), whose infant-directed lateralized behavior might have been specifically selected for as a population-level trait, functional to confer fitness to offspring. In particular, the case of the left-cradling bias (LCB; i.e., the population-level predisposition of mothers to hold their infants on the left side) represents an instance of behavioral trait exhibiting heritability along the maternal line, although no genetic investigation has been carried out so far. Recent evidence, moreover, seems to suggest that the reduction of this asymmetry is related to several unfavorable conditions, including neurodevelopmental disorders. Future studies are warranted to understand whether and how genetic and epigenetic factors affect the lateralization of early mother-infant interaction and the proneness of the offspring to neurodevelopmental disorders.
Highlights
Theoretical models suggest that interactive behaviors are key to the evolution of population-level lateral biases (e.g., Ghirlanda and Vallortigara, 2004): a stable equilibrium in the asymmetrical distribution of lateralized behavioral phenotypes of a Laterality and Neurodevelopmental Disorders given species might be reached through the fitness contribution of both antagonistic and synergistic interactions occurring among its members (Ghirlanda et al, 2009)
Structural asymmetries of the brain are but a small fraction of the Bauplan of neural lateralization—the largest part being expressed in the form of functional asymmetries—and they consist in the allocation of different roles to two structurally similar brain hemispheres (Corballis, 2017)
It must be noted that motor functions deserve a special place in this list, because of the peculiar status of handedness as a function that is lateralized both behaviorally and neurologically from early childhood (Bondi et al, 2020): around 90% of humans show a preference for using the right hand, which is controlled by the left brain hemisphere (McManus, 2002; Tommasi, 2009)
Summary
Behavioral laterality and hemispheric asymmetries are part of a complex biobehavioral system in which genetic factors have been repeatedly proposed as developmental determinants of both phenomena. We suggest the existence of a sensitive period in early postnatal development, during which the exposure to postural and motor lateral biases, expressed in interactive sensorimotor coordination with the caregiver, canalizes hemispheric lateralization in the “typical” direction. Despite newborns and infants showing their own inherent asymmetries, the canalizing effect of the interactive context owes most to adult caregivers (usually the mother), whose infant-directed lateralized behavior might have been selected for as a population-level trait, functional to confer fitness to offspring. Future studies are warranted to understand whether and how genetic and epigenetic factors affect the lateralization of early mother-infant interaction and the proneness of the offspring to neurodevelopmental disorders
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