Early Steps, Early Struggles: Independent Locomotion and Aggression in 4- to 15-Month-Olds.

Although physical aggression emerges in infancy, concurrent with motor development, its possible connection remains poorly understood. Previous research has documented age-related changes in aggression but has not examined how fundamental motor transitions relate to these changes. This investigation examined associations between two key motor milestones (crawling and walking) and aggressive behavior patterns in early childhood. Mothers (N = 377) of infants aged 4-15 months reported on their children's locomotor abilities, aggressive behaviors (directed and exploratory), defiance, and temper loss. Results showed distinct developmental trajectories for different forms of aggression. The prevalence of directed aggression (e.g., hitting, kicking) increased linearly with age, and exploratory aggression (e.g., biting, hair-pulling) followed a curvilinear pattern, plateauing around 9 months of age before declining. Frequency patterns also differed. Exploratory aggression frequency declined linearly with age and directed aggression frequency decreased initially before trending upward around 11 months. The transition to independent walking, but not crawling, was associated with increased frequency of both types of aggression. These effects were mediated through increases in both defiance and temper loss; the indirect effect through temper loss was significantly stronger for directed than exploratory aggression in single-mediator models. These findings suggest that the transition to walking may serve as a reorganizing force in early aggressive behavior in infancy by influencing emerging behavioral opposition and negative emotional expression. Results contribute to a small but growing literature on the developmental systems that underlie aggression in infancy.