Understanding the neurobiology of social behaviour in mammals has been considerably advanced by the findings from two species of vole, one of which is monogamous and pair bonds whereas the other species is promiscuous and fails to form any long-lasting social relationships. The combination of neurobehavioural studies and molecular genetics has determined behavioural differences between the two species linked to the neural distribution of vasopressin 1A receptor in the male brain. More importantly, vasopressin 1A receptor gene transfer including the upstream regulatory sequence has enhanced male social affiliation in a non-monogamous species. Male affiliative bonding depends upon release of both vasopressin and dopamine in the ventral striatum enhancing the reward value of odour cues that signal identity.