Individuals’ development is a multilayered affair. The influence of family relationship on personality, such as Sulloway’s model (1996, 2001) focusing on birth order, is subject to influence from other social systems in which the families are situated. The current research examined the relation of birth order to personality and life satisfaction in China, where only children have become the majority because of national policy. Across two studies with both between-family data (N = 1468) and within-family data (N = 171), onlyborns and laterborns surpassed firstborns on openness to experience. In addition, onlychild participants were more satisfied with their own lives than were sibling participants, especially laterborns. The results offer new insights into the dynamic relations between ecology and personality.