OXTR and major depressive disorder: The role of peripheral oxytocin and markers of the oxytocin receptor gene (OXTR) — the gene that guides the oxytocin receptor — on social affiliation, trust, and mood has been studied extensively in humans, (Bakermans-Kranenburg et al. 2014; Feldman et al. 2011)  suggesting that it may also be involved in clinical depression (IsHak et al. 2011).