This study investigates the trajectories of physical aggression in early childhood and identifies antecedents of high levels of physical aggression. A random sample of 572 families with newborns was recruited, and assessments of physical aggression frequency were obtained from mothers at 17, 30, and 42 months after birth. Using a semiparametric mixture model, three distinct trajectories of physical aggression were identified: low or no aggression (28%), modest aggression (58%), and high aggression (14%). Multivariate logit regression analysis was used to identify family and child characteristics that predict high levels of physical aggression. Key predictors included having young siblings, mothers with high levels of antisocial behavior before high school, early motherhood, low family income, and mothers who smoked during pregnancy. Mothers' coercive parenting behavior and family dysfunction at 5 months of age were also significant predictors. The findings suggest that preventive interventions should target families with high-risk profiles, particularly those with young mothers, early childbearing, and low income. The study highlights the importance of early intervention to prevent chronic physical aggression in childhood, which can lead to serious violent behavior in adolescence and adulthood.This study investigates the trajectories of physical aggression in early childhood and identifies antecedents of high levels of physical aggression. A random sample of 572 families with newborns was recruited, and assessments of physical aggression frequency were obtained from mothers at 17, 30, and 42 months after birth. Using a semiparametric mixture model, three distinct trajectories of physical aggression were identified: low or no aggression (28%), modest aggression (58%), and high aggression (14%). Multivariate logit regression analysis was used to identify family and child characteristics that predict high levels of physical aggression. Key predictors included having young siblings, mothers with high levels of antisocial behavior before high school, early motherhood, low family income, and mothers who smoked during pregnancy. Mothers' coercive parenting behavior and family dysfunction at 5 months of age were also significant predictors. The findings suggest that preventive interventions should target families with high-risk profiles, particularly those with young mothers, early childbearing, and low income. The study highlights the importance of early intervention to prevent chronic physical aggression in childhood, which can lead to serious violent behavior in adolescence and adulthood.