How do toddlers evaluate defensive actions toward third parties?

Defensive behavior is a central aspect of social life and provides benefits to the self and others. Recent evidence reveals that infants evaluate third parties’ prosocial and antisocial actions. Three experiments were carried out to assess toddlers’ reactions to defensive and non-defensive events (N = 54). In two experiments, infants’ looking times and manual choices provided converging evidence that 20-month-olds understand and evaluate defensive actions, by showing that they prefer the defensive puppet over the non-defensive puppet and that they reason on the bystander puppet's disposition. In the third experiment, toddlers rewarded the defensive puppet rather than the non-defensive puppet, revealing how their evaluations guided awarding behaviors of defensive actions toward the third party. The results support the developmental stability and provide evidence of a rich and well-organized prosociality that before the second year of life proves to be based on some moral principles and linked with a sophisticated psychological reasoning. The findings shed light on the claims that human capacities for the social evaluation of defensive behaviors toward third parties are rooted in evolved cooperative systems.