["Journal of Personality, EarlyView. ", "\nABSTRACT\n\nObjective\nPsychopathy is associated with harmful behaviors in romantic relationships, but its causal effect on partner well‐being remains elusive. Across two studies, we apply robust observational methods for causal inference with national‐scale dyadic data to examine how psychopathic personality and its facets affect partner well‐being over time.\n\n\nMethod\nIn Study 1 (three waves; N = 1012), we examined how increasing global psychopathy and its facets within individuals affects their partner's well‐being one year later. In Study 2 (six waves; N = 1832), we investigated how increasing global psychopathy and its facets over multiple years would affect partner well‐being.\n\n\nResults\nStudy 1 indicates that psychopathic personality increases partners' perceived relationship conflict and anxiety in the short term but also bolsters partner self‐esteem. Study 2 reveals that, over the longer term, psychopathic personality intensifies partners' perceived relationship conflict and depression and reduces partner self‐esteem, relationship satisfaction, and personal well‐being.\n\n\nConclusions\nTaken together, it is the convergence of psychopathic traits, rather than individual facets, that cause greater harm to partners, with the magnitude of harm intensifying over time. This research demonstrates the power of combining dyadic panel data with causal inference methods to address questions of psychological interest inaccessible to experiments.\n\n"]