Killing of unrelated young by sexually naïve male mammals is taxonomically widespread, but in many species, males subsequently show paternal care or at least do not harm their own young. This dramatic and important change is due to a shift in paternal state rather than to recognition of young, the mother or the location in which mating occurred. This transition from infanticidal to paternal behaviour is timed so that the inhibition of infanticide is synchronized with the birth of their own young. Ejaculation followed by cohabitation with the pregnant female causes this transition, but the precise stimuli from the female remain elusive. However, changes in social status also cause changes in infanticide. The switch from infanticide is accompanied by physiological change in the male that can be detected by both females and pups. Hormonal changes have been implicated in the switch but establishing causal links has been difficult. Recent neuroanatomical studies show that pup odours activate the vomeronasal organ and its efferent projections to induce infanticide. The emergence of paternal care depends on the inability of the vomeronasal organ to detect pup odours. In the absence of vomeronasal input, pup odours activate a conserved parental circuit and induce caregiving behaviour. An emerging picture is of complex, antagonistic circuits competing for behavioural expression, which allow for males to commit infanticide when they may benefit from such activity but ensure that they do not damage their fitness by killing their own young. However, we stress the need for more work on the neural mechanisms that mediate this process.