Recent models of personality development have emphasized the role of the environment in terms of selection and socialization effects and their interaction. Our study provides partial evidence for these models and, crucially, extends these models by adding a person variable: Core beliefs, which are defined as mental representations of experiences that individuals have while pursuing need-fulfilling goals. Specifically, we report results from a longitudinal investigation of the development of trait greed across time. Based on data from the German Personality Panel, we analyzed data on 1,965 young adults on up to 4 occasions, spanning a period of more than 3 years. According to our results, negative core beliefs that have so far been proposed only in the clinical literature (e.g., being unloved or being insecure) contributed to the development of trait greed, indicating that striving for material goals might be a substitute for unmet needs in the past. Additionally, greedy individuals more often self-selected themselves into business-related environments, which presumably allow them to fulfill their greed-related need to earn a lot of money. Our results expose important mechanisms for trait greed development. Regarding personality development in general, core beliefs were identified as an important variable for future theory building.