Longitudinal Relationship of Early Postnatal Testosterone and Harsh Parenting at 1-3 Months of Age to Physical Aggression at 12 Months of Age in Boys and Girls.

Background: It has been postulated that the early postnatal period, 1 to 3 months of age, is a critical period when early androgen exposure exerts long-lasting influences on aspects of behavioral development that show sex differences. The present study conducted the first test of the relationship between testosterone concentrations at 1 to 3 months of age and subsequent physical aggression. The present study is also the first to examine early postnatal testosterone and harsh parenting simultaneously as predictors of subsequent physical aggression. Methods: The longitudinal sample included 217 boys and 208 girls and their parents. When children were 1 to 3 months old, 3 weekly saliva samples were collected from each child for testosterone assays, and a parent-reported measure was used to assess harsh parenting. When children were 12 months old, parents and children were invited to take part in a follow-up where each child's physical aggression was assessed using both an observational paradigm and a parent-reported measure. Results: There were positive associations between early postnatal testosterone and subsequent physical aggression outcomes in boys and in girls. Also, in boys, early postnatal testosterone and harsh parenting independently and interactively predicted subsequent parent-reported physical aggression; there was a positive association between testosterone and aggression when harsh parenting was high but not when harsh parenting was low. Conclusions: The current findings suggest that early postnatal testosterone may exert organizing influences on physical aggression development in boys and in girls. Programs designed to reduce harsh parenting may buffer these early hormonal influences, especially in boys.