Even though it is also clear that endogenous opioids are very robust mediators of similar psychological processes, we here focus on oxytocin because of the great potential for immediate applications to various human clinical problems ranging in adults (from addiction to depression to suicidality; Stevens et al., 2013; Yovell et al., 2016) and in child development (Quattrocki and Friston, 2014). The gene discussed is OXT; the disease is major depressive disorder.