What This project aims to reveal the underlying mechanisms by which the neurohormone oxytocin, involved in parenting, pair bonding, and social memory, contributed to the evolutionary origins of joint music making. By testing for the first time whether performing music with others leads to increased oxytocin levels, we hope to discover a key missing link in our groundbreaking theory of a hormonal feedback loop whereby musical interaction increases oxytocin which in turn enhances synchronisation. This would offer the most convincing evidence to date for the musical "grooming hypothesis", thus unifying efforts in evolutionary psychology, musicology, and neurobiology explaining music as a complex, socially embedded, and cross-culturally universal phenomenon, distinguishing us from other species. Why Joint music making is omnipresent in human cultures with possible origins far beyond the established 40,000 years of archaeological evidence. Yet, we do not know why humans make music together or what makes musicality biologically adaptive. Inspired by the significance of music in sports, war, and religion, influential evolutionary theories recognise music's potential as a "biotechnology of group formation". The "grooming hypothesis", in particular, predicts that joint singing and drumming enabled consolidation of social unity amongst homo sapiens whose increasing group sizes rendered physical grooming unfeasible. Despite behavioural findings that coordinated actions promote trust and affiliation, direct evidence for the neurobiological underpinnings of this hypothesis remains missing. How To test the hypothesis that joint music making increases oxytocin levels to a larger extent than solitary music making and passive listening, salivary samples will be self-collected by 25 pairs of musicians before and after singing/playing together in duets or singing/playing alone while being observed by their duet partner. This within-participant design matches musical repertoire, instrumentation, partner presence, and individual differences in personality and neurohormones across conditions while maintaining high ecological validity. Salivary oxytocin will be quantified by radioimmunoassay following established, validated procedures, and expert musicians will be recruited from the music program at University of New South Wales. SSR Given the clinical use of purified oxytocin (e.g., to induce childbirth and treat breast engorgement in lactating mothers), a deeper understanding of the underlying mechanisms and effects of oxytocin on central brain processes gained from basic research may inform future biomedical treatment regimens. Causally linking joint music making to oxytocin secretion may pave the way for non-pharmacological interventions with limited side effects and increased patient enjoyment. More broadly, solving the evolutionary riddle of why we make music may enrich our understanding of the human condition. This provides evidence-based knowledge to political decision-making on the role of music in schools, healthcare, community building, and care for the well-being of children, adults, and the elderly.