What did selective-breeding studies in mice, crickets, and sticklebacks show about aggression?

Selective-breeding studies in mice, crickets, and sticklebacks have demonstrated that aggression is influenced by genetic factors. In these species, researchers found that selective breeding for high or low levels of aggression in males produces a marked and rapid response. This indicates that at least some of the original variation in aggressiveness within a parental population is the result of underlying genetic differences.1

These findings highlight that while environmental factors—such as social experience, developmental conditions, and hormonal exposure—play a significant role in the development of aggressive behaviour, there is a clear genetic component to an individual's predisposition toward aggression. Ultimately, how an animal behaves as an adult is not the result of a single factor but rather a continuous and complex interaction between inherited genetic material and the environment in which those genes are expressed.12