Sex differences in the activity level of infants

A gender difference in motor activity level (AL) is well established for children, but questions about the existence and nature of an infant sex difference remain. To assess these questions, we applied meta-analytic procedures to summarize 46 infancy studies comprising 78 male–female motor activity comparisons. Our results showed that, as with children, male infants were more active than females. Objective measures of infant AL estimated the size of this difference to be 0.2 standard deviations, though subjective parent-report measures estimated the difference to be smaller. We argue that this early sex difference in activity level is biologically based. However, socialization processes, such as gender-differentiated expectations and experiences, in conjunction with further sex-differentiated biological developments, amplify this early difference to produce the larger gender differences in activity found during childhood.