ABSTRACT On the basis of the assumptions of the dynamic transactional paradigm, the current study investigates the effects of success and failure of social investment on personality development across young adulthood. Using longitudinal data from the Michigan Study of Adolescent and Adult Life Transitions (MSALT), the authors demonstrate that entering into the first long-term romantic relationship was accompanied by decreases in facets of neuroticism, thereby fully replicating findings by Neyer and Lehnart (2007) based on a German longitudinal study. In addition, remaining single over 8 years was related to decreasing self-esteem, especially for men. These results demonstrate long-term effects of investment and lack of investment in social roles and provide further evidence of the interrelatedness of social or relationship experiences and personality development.