Jane Coryell

How supine postural preferences of infants can contribute toward the development of handedness

During their first three months postpartum, infants manifest an asymmetrically lateralized head position preference, typically turned to the right. This head position preference elicits an asymmerical tonic neck reflex, which places one hand in the infants visual field. As a result, infants have differential visual experience of their two hands. The majority of infants have more visual experience with their right hands than their left. Knowledge of which hand an infant has had more visual experinece of, as a result of its postural preference, reliably predicts the hand that will be used most in a visually-elicited reaching task at 12 weeks postpartum. Therefore, the origin of human handedness status may reside in an asymmetrical postural preference during early infancy, which biases visual experience of the hands, giving one hand an advantage in eye-hand coordination tasks.

Infant rightward asymmetries predict right-handedness in childhood

Eight children for whom measures of head preference and hand visualization had been determined in infancy were evaluated by parent questionnaire for hand preference at ages 3.5-6 yr. All six infants who maintained their heads to the right became right-handed as predicted, while neither infant with a non-right head orientation developed handedness in the predicted direction, though neither became right-handed. Similarly, the five infants who had visualized their right hands more frequently than the left became right-handed children as predicted; of the three infants who did not visualize the right hand more frequently, none developed handedness as predicted. None of the infant measures correlated with childhood handedness. These results suggest support for the genetically based right shift factor proposed by ANNETT (Behav. Genet. 8, 227-249, 1978).