The studies focused on two species that look nearly alike: prairie voles and montane voles. The first time a male prairie vole mates with a female, he forms a bond with her for life, breeding and raising successive litters. Male montane voles think of sex as a series of one-night stands; they are loners and do not bond with females or help raise offspring.

Young and others concluded that the difference between these species is because of the same gene variant that Walum studied. In the kind of experiments that cannot be replicated in humans, Young and others also showed that manipulating vasopressin receptors in vole brains can turn loner voles into devoted partners and fathers, and vice versa.