Implications of sex differences in neonatal temperament for early risk and developmental/environmental interactions

Sex differences in neonatal temperament were examined in 30 pairs of full-term, opposite-sex twins. Behavior was assessed during feeding, during sleep, while awake for orienting responses, in response to a stressor, and for measures of spontaneous irritability and soothability. Composite and summary scales described irritability, resistance to soothing, reactivity, reinforcement value of the infant's behavior, activity during sleep, and activity while awake. Analyses of variance of paired comparisons performed separately for each behavioral area indicated that female neonates were more irritable and more difficult to soothe than male neonates. The results are discussed in terms of maturational effects on neonatal behavior, implications for infant-caregiver interaction, and their relation to environmental influences on development.