The interrelationship between personality traits and major depressive disorder during adolescence and early adulthood.

University of Minnesota Ph.D. dissertation. July 2012. Major: Psychology. Advisor: William G. Iacono. 1 computer file (PDF); ix, 84 pages.Background: Research literature has documented a relationship between personality traits and depression. However, no prospective studies have explored the influence of depression on personality development during adolescence and young adulthood. Objective: The purpose of this dissertation is to examine how depression affects personality development, and conversely, how personality traits confer risk for depression using the context of normative developmental change in adolescence and early adulthood as a frame of reference. Method: Participants included twins from the 11-year-old and 17-year-old cohorts of the Minnesota Twin-Family Study (MTFS). In order to assess the bidirectional influence between depression and personality, groups were created based on age of depression onset and course. Personality was assessed using scales from the MPQ, a personality instrument designed to assess personality characteristics in normal populations. The impact of the onset of depression on each personality variable was examined using linear mixed models (LMM) in SPSS. Results: In the first study, Negative Emotionality (NEM) in mid-adolescence acted as a vulnerability factor, and was associated with the onset of depression. Greater levels of Stress Reaction and Alienation were associated with an earlier onset persisting course; later onset of depression was associated with an increase in these traits. In mid-adolescence never depressed individuals scored higher than all groups in Well-Being. The onset of depression corresponded to a decrease in WB, while remission was associated with an increase in WB. In the second study, elevated NEM predicted the development of new cases of MDD in late adolescence and young adulthood. In those with adolescent onset depression, low Positive Emotionality (PEM) was associated with persisting course. Late onset and earlier persisting depression were associated with PEM decreasing from age 17 to 24. Low Constraint (CON) in adolescence was associated with a persisting course. In the third study, greater levels of NEM were associated with the onset of depression, suggesting NEM indexes underlying vulnerability. Earlier onset persisting depression slowed down the normative age-related decrease in NEM. Recurring depression was associated with lower levels of PEM. Higher PEM was associated with remission. CON had no effect on the onset and course of depression. Conclusion: NEM predicted the subsequent onset of MDD, was moderately influenced by clinical state, and influenced the course of depression. Findings for CON were weak and inconsistent. PEM did not act as vulnerability. However, lower levels of PEM were associated with persisting depression, while higher levels were associated with remission. To conclude, the association between MDD and personality varies with the course of MDD, and indicates both that personality is relevant to prognosis and that the course of MDD may alter personality