Goal-directed social behaviours such as mating and fighting are associated with scalable and persistent internal states of emotion, motivation, arousal or drive. How those internal states are encoded and coupled to behavioural decision making and action selection is not clear. Recent studies in Drosophila melanogaster and mice have identified circuit nodes that have causal roles in the control of innate social behaviours. Remarkably, in both species, these relatively small groups of neurons can influence both aggression and mating, and also play a part in the encoding of internal states that promote these social behaviours. These similarities may be superficial and coincidental, or may reflect conserved or analogous neural circuit modules for the control of social behaviours in flies and mice.