oxytocin-enhanced prosocial behaviour depends on individual characteristics. this study investigated the relationship between oxytocin and competitiveness, which is another important social trait and predicts economic and social outcomes. in this double-blind, randomized, and placebo-controlled study of 192 male participants, we examined whether oxytocin moderates competitiveness and whether the effect of oxytocin on competitiveness is amplified in individuals with autistic traits. While our results show no relationship between oxytocin and competitiveness, we observed suggestive patterns: albeit not significantly, oxytocin reduced and enhanced competitiveness among participants without autistic traits and among their counterparts with autistic traits, respectively. While oxytocin reportedly enhances prosocial behaviours such as trust 1 , generosity 2 , and parochial altruism 3 , recent studies have shown this effect to depend on context and individual characteristics 4 like autistic traits 5 and baseline oxytocin levels 6. Hirosawa et al. 5 found that oxytocin induces prosocial behaviours in individuals with traits associated with autism spectrum disorder (ASD) 7-i.e., low empathy quotient (EQ) 8 , high systemizing quotient (SQ) 9 , and high autism spectrum quotient (AQ) 10 scores 11-13. It is important to investigate for whom and in what aspects oxytocin can be effective, because oxytocin has clinical implications for people with ASD, who commonly show defects in social behaviour 14. The current study expands the literature on the behavioural effects of oxytocin in two ways. First, we tested whether oxytocin moderates the scantly investigated but important social trait of competitiveness; while the oxytocin literature shows a relative dearth concerning the potential effect of oxytocin on competitiveness, recent social science research finds that competitiveness can predict important economic and social outcomes 15-17. Second, we tested whether the effect of oxytocin is amplified in individuals with autistic traits. Experimentally, competitiveness can be measured according to a paradigm developed by Niederle and Vesterlund 18 : individuals choose a payment scheme between a piece-rate and a tournament environment. While the piece-rate environment is non-competitive, the tournament environment is competitive because one's payment depends on relative group performance. The tournament represents labour market outcomes, such as promotions and hiring. A growing body of literature focuses on the effects of external factors such as environment 19-21 , culture 22,23 , and internal factors such as hormones 24-27 and preferences 28,29 on competitiveness. Bartling et al. 28 found that individuals with higher personality trait scores for "agreeableness" were less likely to enter tournaments than to enter piece-rate schemes. These findings suggest a negative association between competitiveness and agreeableness, which has been found to relate to cooperative preferences 30 and prosocial behaviours, including trust and altruism...