The evolutionary perspective on human emotions and motivations posits that all interpersonal interactions are shaped by an array of social mentalities, dwelling on our species' bio-behavioral disposition to pursue some evolutionarily valuable social goals (i.e., interpersonal motivational systems). The paucity of valid and reliable measures of such mentalities has limited empirical research into how these processes play out in everyday social exchanges. The Social Mentalities Scale (SMS) was developed to evaluate patterns of cognition, affect and behavior from basic interpersonal motivational systems. Two samples of young adults (18–35 years old) completed distinct instrument packets including the SMS and self-report questionnaires. An exploratory factor analysis (740 participants) revealed a six-factor solution: insecurity, prosociality, agonism, belongingness, sexuality, and playfulness. A confirmatory factor analysis (815 participants) supported the goodness of this factor model. Moreover, the SMS's subscales were correlated to specific dimensions of individual psychological functioning in a theoretically coherent way. These results supported the SMS's validity and reliability in assessing the complex and multifaceted portrait of social mentalities that inform human interactions and personality. The SMS is a user-friendly and easy to complete measure that promises to provide a significant contribution in a potentially wide range of clinical and research contexts.