Personality development in young adulthood has been associated with the experience of a number of new social roles. However, the causal interpretation of these findings is complicated by the fact that it is not possible to randomize young adults by their life experiences. To address this problem in the context of the first partnership experience, we applied propensity score matching to a sample of initially inexperienced singles and followed them across 4 years. Using matched samples, results indicated that the first partnership experience relatively robust increased life satisfaction. The first partnership experience between the ages of 23 and 25 (but not in other ages) was also related to higher self-esteem, extraversion, and conscientiousness and to lower neuroticism. The discussion highlights the effect of the first partnership on the development of a mature personality and the potential for propensity score matching to make useful contributions to social and personality research.