Sexual conflict over reproduction is widespread, and can lead to the evolution of traits in males that enhance their reproductive success but harm the females they mate with. I addressed the broad question of whether the social environment, and in particular relatedness among competing males, is important for the outcome of sexual conflict. I found that male seed beetles plastically adjusted their harmfulness in response to changes in both competitor relatedness and chemical cues to kinship. The conditions that allow non-plastic divergences in harmfulness, however, are more restrictive. The social environment is therefore important for male harmfulness and female fitness.