The broad autistic phenotype (BAP) is a constellation of traits that mirror those of clinical autism and share the same mechanisms as the condition. Social, communicative, and rigid/repetitive BAP traits can therefore be studied in the general population. Socio-communicative BAP traits are defined according to broad diagnostic criteria for autism, comprising multiple potential neural mechanisms. Autism studies have suggested various differences in socially relevant brain networks for e.g., face processing, mentalizing, and social reward. This study aimed to stratify the BAP into more specific constructs relevant for social function, in line with the Research Domain Criteria (RDoC). We recruited 148 adults with a broad range of autistic traits to a behavioral study combining face perception tasks and self-report measures. We measured the classical BAP through the Comprehensive Autistic Traits Inventory and quantified the RDoC-style constructs of social anhedonia (SA), mentalizing, prosopagnosic traits (face blindness) and lower-level face matching performance. We found that social anhedonic and prosopagnosic traits strongly and independently predicted socio-communicative BAP traits, suggesting roles for reward systems as well as face processing networks in autistic social function. Objectively measured face matching sensitivity was also negatively correlated with the BAP, but the effect size was smaller in comparison to prosopagnosic traits which also depend on face memory. Mentalizing (attributing mental states to images of eyes) showed specific correlations with low-level face perception and self-reported prosopagnosic traits, but not with the BAP. This study supports prosopagnosic and anhedonic traits as relevant constructs for future studies on neural mechanisms of social difficulties in autism.