Human prosocial behavior emerges in the 2nd year of life, posing challenging questions about mechanism. An increasingly common claim is that prosocial behavior in the first 3 years of life is neither a result of nor affected by socialization. In this article, we argue that early socialization plays a critical role in the developmental origins of prosocial behavior. To support this argument, we highlight conceptual perspectives and empirical evidence for influences of socialization from parents' reports, direct observation, and experimental studies. We conclude that progress in explaining the ontogenetic origins of human prosocial behavior depends on more fully specifying mechanisms of socialization in infancy as well as what and how human infants learn from their social experiences.