This chapter examines the history of American personality psychology, focusing on a puzzling question: In a field defined as the scientific study of individual persons and differences among persons, how can we explain the history of ambivalence toward case studies and life histories? Adopting a multidisciplinary perspective, we suggest that the marginal status of these methods in personality psychology was related to their identification between 1900 and 1930 as preferred methods in psychiatry, psychopathology (considered primarily a medical specialty at the time), and sociology. In contrast, psychologists interested in personality adopted psychometric measures that could establish their scientific expertise in meeting practical goals. These preferences persisted during the emergence of personality psychology as a separate subdiscipline in the 1930s, despite calls for more attention to case study methods. To illustrate the lasting effects of these methodological choices, we trace the efforts of Gordon Allport and Henry Murray to promote the study of individual lives in personality psychology, and we examine psychologists' responses to their work. Finally, we reconsider the historical course of personality psychologists' ambivalence toward the study of individual lives and suggest an interpretation of the revival of interest in case studies, life histories, and psychobiography in recent years.