An important debate at the intersection of personality, developmental, and social psychology is whether, because of large variability and situation-responsiveness within each person's behavior, research should focus on individual differences in reactions to situations, or because of stability in the way the same person acts over time, research should focus on traits. The proposed research tests the hypothesis that both are needed and in fact clarify each other, as specified in the following 3 aims: (1) Employ independent behavioral data to test the hypothesized integration of process and structure; (2) Determine the amount of variability across the life span; and (3) Chart the contingencies and consequences of within-person variability, individual differences in variability, and age differences in variability. Behavioral variability and the contingencies of variability are at the heart of several conceptions of mental health. Personality disorders such as borderline or histrionic may be characterized by unmanageably high variability whereas others such as schizoid, avoidant, or obsessive-compulsive may be characterized by rigidly low variability. Some personality disorders may further be characterized by maladaptive contingencies of variability, such as the strong reaction to criticism in narcissism, the strong reaction to neglect in borderline disorder, or the limited reaction to danger in antisocial personality disorder. Mental health theory would be aided by understanding of the basic processes underlying behavioral flexibility. Furthermore, recent research has linked personality disorders to the traits of the Big Five (5). The proposed process model that explains the behavioral manifestation of traits in reaction to situations may help clarify the mechanisms of these links and also identify) opportunities for intervention. Finally, the proposed research will explore when flexibility becomes adaptive and when it becomes maladaptive by correlating individual differences in flexibility and in the contingencies of flexibility with various indicators of mental health. The research in this application is proposed because it may establish an approach to personality that moves personality forward, past the person-situation debate in an integrative manner, because it helps validate 2 important methodological tools for psychology, mental health, and health research (experience-sampling methodology and self-report) and because it investigates within-person behavioral variability, which is at the heart of self regulation and many personality disorders.