Paul T. Costa Jr. is a prominent American psychologist known for his significant contributions to the field of personality psychology. He is best recognized for his work on the Five Factor Model of personality, which outlines five broad dimensions that describe human personality traits. His research has helped shape our understanding of how personality influences behavior and mental health. Costa has collaborated extensively with other researchers, including Robert R. McCrae, to develop assessments that measure personality traits. Their work has been influential in both academic and clinical settings, providing valuable tools for psychologists and researchers to better understand individual differences in personality.