ABSTRACTSocial interaction allows for the transfer of affective states among individuals, and the behaviors and expressions associated with pain and fear can evoke anxiety-like states in observers which shape subsequent social interactions. We hypothesized that social reactions to stressed individuals engage the serotonergic dorsal raphe nucleus (DRN) which promotes anxiety-like behavior via postsynaptic action of serotonin at serotonin 2C (5-HT2C) receptors in the forebrain. First, we inhibited the DRN by administering an agonist (8-OH-DPAT, 1µg in 0.5µL) for the inhibitory 5-HT1Aautoreceptors which silences 5-HT neuronal activity via G-protein coupled inward rectifying potassium channels. 8-OH-DPAT prevented the approach and avoidance, respectively, of stressed juvenile (PN30) or stressed adult (PN60) conspecifics in the social affective preference (SAP) test in rats. Similarly, systemic administration of a 5-HT2Creceptor antagonist (SB242084, 1mg/kg, i.p.) prevented approach and avoidance of stressed juvenile or adult conspecifics, respectively. Seeking a locus of 5-HT2Caction, we considered the posterior insular cortex which is critical for social affective behaviors and rich with 5-HT2Creceptors. SB242084 administered directly into the insular cortex (5µM in 0.5µL bilaterally)interfered with the typical approach and avoidance behaviors observed in the SAP test. Finally, using fluorescentin situhybridization, we found that 5-HT2Creceptor mRNA (htr2c)is primarily colocalized with mRNA associated with excitatory glutamatergic neurons (vglut1) in the posterior insula. Importantly, the results of these treatments were the same in male and female rats. These data suggest that interactions with stressed others require the serotonergic DRN and that serotonin modulates social affective decision-making via action at insular 5-HT2Creceptors.