Summary of work: Most studies of age differences in personality have been conducted in the United States using self-report measures. For this project, two studies were conducted across cultures using observer ratings of personality. In the first, we obtained both self-reports and informant ratings of the same individuals in Russia (N = 800) and the Czech Republic (N = 705). In both cultures, the same pattern of age differences was found: Neuroticism, Extraversion, and Openness declined across the lifespan, whereas Agreeableness and Conscientiousness increased. Informant data generally replicated self-report findings, although the effects were weaker. In the second study we gathered college students' ratings of an anonymous target, either 18-21 or 40+ years old, in 50 cultures, from Iceland to Ethiopia. Adults were rated as higher than college-age targets on Conscientiousness and lower on Extraversion and Openness in most cultures. Age differences on Neuroticism and Agreeableness were much weaker. Although there are still some unexplained differences between self-report and informant rating methods of assessing personality, the data overall demonstrate that age differences are universal, supporting the hypothesis that personality change is part of the intrinsic maturation of the human species.