Genetic effects on infant handedness under spatial constraint conditions

Previous studies have reported a genetic influence on the individual differences in adult handedness; however, relatively little is known about genetic influences on the development of infant hand selection. In the current study, we examined whether genetic influences on handedness are expressed in various spatial locations in infants aged 18 months using the twin method. Infants were asked to respond to targets positioned in left, middle, and right locations using grasping movements. Results showed that similarities in hand selection within monozygotic twin pairs was more than two times higher than that of the dizygotic twin pairs in the middle location. In the left location, similarities in hand selection within monozygotic and dizygotic twin pairs were low. In addition, low individual differences in hand selection in the right location were also observed. These results suggest a non-additive genetic influence on handedness, and that spatial constraint is a crucial factor for the expression of genetic effects on handedness in infants.