Stability of individual differences of neonatal motor and crying patterns.

Longitudinal research suggests that characteristic levels of individual energy expenditure are enduring over time and influencing personality development. To measure characteristic levels of energy output at the very beginning of life, the activity and crying patterns of 72 normal neonates were monitored for 1-3 days by an electronic activity monitor. This monitor measures crying, noncrying, and total activity of various amplitudes and cry time. Day-to-day stability was highly significant in daytime crying, noncrying and crying activity, and median amplitude of noncrying movements. The amplitude of noncrying movements had the highest day-to-day stability, with correlations of .69-.80 This measure of the vigor of the infants' movements was not related to the infants' activity level as expressed in the frequency of movements and was not confounded by the infants' crying. The activity and crying measures were not significantly affected by the infants' sex or weight, length of labor, maternal parity, or by the analgesic drugs used during labor and delivery. Over the first 3 days of life, the amplitude or vigor of the infants' movements increased, while the frequency of motions did not change significantly.