Humans exhibit socially contagious behaviours, including contagious itching, but studying the underlying mechanisms has been difficult. Yu et al. found that they could socially transmit scratching to mice by allowing them to watch mice with chronic itch. The observer mice showed upregulated neuronal activity in the suprachiasmatic nucleus (SCN) after watching the mice that exhibited excessive scratching. Ablation of gastrin-releasing peptide (GRP) signalling in the SCN inhibited scratching transmission but not the ability of the mice to scratch, and optogenetic activation of GRP-expressing SCN neurons induced scratching. These data provide evidence for a circuit underlying a socially contagious behaviour in mice.