Aggression experience and observation drive shared behavioural and neural changes.

The ability to observe the social behaviour of others and use observed information to bias future action is a fundamental building block of social cognition 1-3 . Aggression observation activates hypothalamic neurons relevant for aggression control 4,5 , but whether observation and experience engage common circuit mechanisms that promote a new shared behavioural policy is unclear. Here we develop a strategy to perform large-scale cell-type-specific recordings across subcortical networks for social behaviour generation and learning. We record longitudinally while animals train through direct aggression experience or observation, then probe shared differences in behaviour and neural activity in a novel aggression context. After training, we detect unique signatures of a shared adaptive behavioural strategy that is absent in animals without training. During aggression observation, observed behaviour recruits widespread activation across subcortical networks for behavioural generation and mimics activity evoked by experienced behaviour, and shared neural changes persist into the novel aggression context. Multiregion modelling reveals shared changes to a core aggression network across both experience and observation, including a widespread reduction in inhibition, which we find is necessary for adaptive behavioural change. Together, this demonstrates that experience-like activity during aggression observation can recruit a shared inhibitory plasticity mechanism that biases behaviour toward adaptive proactive behavioural strategies.