Children grow and adapt very obviously and constantly, but they are not the only ones. People change across their whole life span. Empirical evidence shows that also adults’ personality (i.e., openness to experience, conscientiousness, extraversion, agreeableness, neuroticism) and their social participation (e.g., social network size) change up until old age. Even though the phenomenon of lifelong personality trait change is widely accepted, the causes of change and thus the possibilities to intervene are much debated currently. So far, little attention has been paid to the role of social participation with regard to personality change and vice versa. Firstly, this thesis introduces the literature on personality development and social participation over the life span (Chapter 1), followed by a review of the empirical evidence on changes in personality traits and in social behavior, i.e., in-person social contacts, leisure activities (Chapters 2-4). In order to further contribute to research on life span development and catalysts of personality change, the following research questions (RQ) will be investigated: RQ 1. Can an intensive cognitive training change personality trait, especially openness to experience, in the long run? (Study 1, Chapter 2) RQ 2. How does in-person social contact frequency develop across the adult life span? (Study 2, Chapter 3) RQ 3. Are changes in frequency of different leisure activities and overall (social) participation associated with personality change? – If so, which direction do they take? (Study 3, Chapter 4) The data to answer these questions are derived from two data sets. Firstly, the COGITO study that was originally designed to investigate day-to-day fluctuations in cognitive performance and to examine transfer effects of trained cognitive tasks on nontrained cognitive tasks (Chapter 2). Secondly, the German Socio-Economic Panel Study (SOEP), which is a large, ongoing survey of private households and individuals in Germany which was initiated in 1984 (Chapters 3 and 4). The results of the empirical chapters lead to the following answers (A) to the research questions: A 1. Cognitive training does not affect changes in any facet of openness to experience in the long-run. This was true for young and old participants as well as for men and women. A 2. The frequency of in-person contacts with family members remains relatively stable across the life span. The frequency of visits to and from nonfamily members (neighbors, friends, and acquaintances) declines, following a cubic trajectory and drops below the frequency of family visits once people are in their mid-30s. A 3. Frequency of different leisure activities and the overall participation are most strongly associated with openness to experience trait at a between-person level. However, at within-person level, reciprocal effects are discerned only for extraversion with overall participation and socializing (i.e. in-person contacts). These results are integrated into the literature on personality development and social psychology in the general discussion and implications for research and practice are discussed respectively (Chapter 5). In conclusion, this thesis minimizes some gaps in the literature by comprehensively investigating mechanisms of personality change and patterns of social behavior in adulthood.
