The tendency to draw mind-brain dichotomies and evaluate mental disorders dualistically arises in both laypeople and mental health professionals, leads to biased judgments, and contributes to mental health stigmatization. This paper offers a theory identifying an underlying source of these evaluations in social practice. According to this theory, dualistic evaluations are rooted in two mechanisms by which we represent and evaluate the beliefs of others in folk psychology and theory of mind: the doxastic conception of mental disorders and doxastic voluntarism. Tracing these origins contributes to our understanding of mental state representation in cognitive science and philosophy of psychiatry, the concept of belief in philosophy of mind, and may help improve patient experience and treatment in light of social stigmatization and bias toward mental illness.