The effectiveness of current treatment options for sociocognitive deficits and negative symptoms (NS) in schizophrenia spectrum disorders (SSD) remains limited. The cause of NS is thought to be an interference between the mesocorticolimbic dopamine system for social reward expectancy and the network for socioemotional processes. Oxytocin (OXT) may enhance functional connectivity between these neuronal networks. Lower plasma OXT levels correlate negatively with NS severity and deficits in social cognition in SSD. It has been shown that intranasal OXT administration improves social cognition in healthy subjects but in SSD results are inconsistent. According to the social salience hypothesis, the effect of OXT varies depending on the social context and individual factors. Also, OXT-mediated effects on psychopathology and NS may depend on genetic variants of OXT receptors (OXTR). In a pilot study, the investigators demonstrated lower NS by OXT administration in a positive social context of mindfulness-based group psychotherapy (MBGT) in SSD. The investigators also demonstrated that symptoms improved after MBGT. A more recent study suggests that, compared to placebo, administering OXT in a positive social context via MBGT leads to significant between-group differences favoring OXT, particularly in NS, affect, and stress. Building on these findings, the present study investigates the stability of these effects, along with psychological and biological markers, in a larger sample of individuals with SSD. The main hypothesis to be tested is that the use of OXT compared to placebo prior to MBGT in patients with SSD will result in a greater reduction in NS with a higher OXT dosage. The research design is based on an experimental, triple-blind, randomized, placebo-controlled trial.