Social neuroscience has benefitted from the legacy of two venerable traditions: studies of sensory processing from neuroethology and studies of social behaviors from neuroendocrinology. The vast and often mysterious territory in between the sensory input and motor outputs – the “dark matter” – involves integrative circuits that remain to be fully described. At the simplest level, the outputs can be described as approach and affiliation, which Taylor and colleagues have called “tend and befriend” versus agonistic behavior or avoidance, classically called “fight or flight”.()

Receptors are labeled with iodinated ligands by in vitro receptor autoradiography. Levels matched across species with arrows pointing to homologous structures. Prairie voles show higher binding in nucleus accumbens for oxytocin and ventral pallidum for vasopressin. Meadow voles show higher binding for vasopressin in lateral septum. Not shown are differences in other regions including posterior cingulate-retrosplenial cortex (high for vasopressin V1a receptor in prairie vole) and ventral thalamus and amygdala (high for oxytocin receptor in meadow vole). Abbreviations: PFC prefrontal cortex, CP caudate putamen, NAcc nucleus accumbens, LS lateral septum, VP ventral pallidum. Figure adapted from ()

In monogamous prairie voles, pair bond formation usually occurs as a consequence of mating. The model is based on the release of oxytocin and vasopressin with mating. In prairie voles, these neuropeptides activate receptors and interact with dopamine in brain regions associated with reward and reinforcement. The model presumes that the neuropeptides transduce the sensory information about the identity of the mate to a highly salient reinforcer. The pair bond is operationally a conditioned response to the mate. In non-monogamous voles, the same peptides are released with mating but the absence of receptors in reward pathways precludes pair bonding. Experimental evidence supporting this model includes blockade of pair bonding by local administration of antagonists for oxytocin and vasopressin V1a receptors in mating prairie voles and induction of partner preference formation in non-monogamous voles by local viral vector induction of receptors. Abbreviations: OT – oxytocin, AVP – vasopressin, DA – dopamine, VP – ventral pallidum, RS – retrosplenial cortex, AH – anterior hypothalamus, NAcc – nucleus accumbens, D2 –dopamine-2 receptor, LS-lateral septum, Thal – thalamus.

Schematics of genomic structure of V1a and OT receptors show regions of interest for social cognition. Two microsatellites in the 5′ flanking region of the V1a receptor, denoted RS1 and RS3, have been associated with autism (; ; ). In particular, the length of RS3 has been associated with a range of interpersonal skills (reviewed by () as well as several measures of fidelity in men(). The OTR includes many intronic SNPs, with the cluster shown in the first intron linked as a haplotype to autism in Chinese Han (), Japanese (Liu et al.), and Israeli () cohorts. In a Caucasian cohort (), neither this haplotype nor the third intron SNP was associated with autism and in the single positive allele at rs2254298, the G allele was overtransmitted to probands, opposite to the overtransmission of the A allele reported in other populations. The rs53576 SNP in the third intron, which showed the largest effect in a family based association test in the Han Chinese study () has also been associated with measures of parental sensitivity, altruism, and a test of the ability to read the emotional state of others.(; )