Asymmetric action in the human newborn: sex differences in patterns of organization.

We developed a procedure and scale to quantify movement asymmetry in 36 full-term newborns from several normal newborn nurseries (Measure of Behavioral Laterality, MOBL). The majority of newborns had elicited reflexes and spontaneous movements that were stronger and more coordinated on the right than on the left side of the body; there was no asymmetry in the latency, threshold, or habituation of these behaviors. Although asymmetry of different movements was associated if they relied on the same motor pool, there was little association of movement asymmetries among different body regions, indicating that multiple subsystems, rather than a single asymmetric system, controls asymmetric action in the newborn. Finally, there was a sex difference in asymmetry of all 3 distal lower-body elicited reflexes: females were right biased, but the majority of males were left biased. These sex differences are discussed in terms of alternative mechanisms for the development of asymmetric action and the role of newborn reflexes in adult voluntary movement.