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

The prevalence of directed aggression increased linearly with infant age across the 4-15 month range.

• Directed aggression includes behaviors such as hitting and kicking.
• Sample consisted of mothers (N = 377) of infants aged 4-15 months.
• The linear trajectory was distinct from the pattern observed for exploratory aggression.
• Mothers self-reported on their children's aggressive behaviors.

Exploratory aggression prevalence followed a curvilinear developmental pattern, plateauing around 9 months of age before declining.

• Exploratory aggression includes behaviors such as biting and hair-pulling.
• The curvilinear pattern was distinct from the linear increase seen in directed aggression.
• The plateau and subsequent decline occurred around 9 months of age.
• This suggests exploratory aggression may be functionally different from directed aggression developmentally.

Frequency patterns for directed and exploratory aggression differed from each other and from their respective prevalence patterns.

• Exploratory aggression frequency declined linearly with age.
• Directed aggression frequency decreased initially before trending upward around 11 months.
• These frequency patterns were distinct from the prevalence trajectories observed for the same behaviors.
• The divergence suggests that as infants age, directed aggression becomes less common but occurs more frequently among those who engage in it.

The transition to independent walking, but not crawling, was associated with increased frequency of both directed and exploratory aggression.

• Two key motor milestones were examined: crawling and walking.
• Crawling onset was not significantly associated with either type of aggression frequency.
• Walking onset was associated with increased frequency of both directed and exploratory aggression.
• This suggests walking represents a qualitatively different developmental transition than crawling with respect to aggressive behavior.

The association between walking and increased aggression frequency was mediated by increases in both defiance and temper loss.

• Both defiance and temper loss served as significant mediators of the walking-aggression relationship.
• The indirect effect through temper loss was significantly stronger for directed aggression than for exploratory aggression in single-mediator models.
• The mediation analysis suggests walking influences aggression through emerging behavioral opposition and negative emotional expression.
• These findings highlight a developmental systems pathway from motor milestone to social-emotional behavior.

The authors propose that the transition to walking may serve as a reorganizing force in early aggressive behavior in infancy.

• Walking is characterized as influencing 'emerging behavioral opposition and negative emotional expression.'
• The study situates these findings within 'a small but growing literature on the developmental systems that underlie aggression in infancy.'
• The reorganizing effect is theorized to operate through the child's increased autonomy and the consequent social and emotional challenges that arise.
• The findings extend prior age-related aggression research by identifying a specific motor transition as a contributing mechanism.