Socially Transmitted Aggression (STA) is a unique form of observational learning in which mice acquire aggressive behavior after witnessing familiar peers attack others. Our research shows that this process relies on specific brain circuits, including the medial amygdala and ventromedial hypothalamus, which become sensitized by familiarity. By combining advanced neuroscience tools with behavioral models we developed, we are uncovering how peer networks shape aggressive behavior. This work has broad relevance to real-world problems such as adolescent violence, gang dynamics, and the social contagion of harmful behaviors.