Differences in children's self-regulation are assumed to be explained by genetic factors, socialization experiences, and sociodemographic risk. As for socialization, little research has addressed the influence of having siblings or attending early center based child care on emerging self-regulation. As regarding sociodemographic risk, few studies have been conducted in countries characterized by high equality and little poverty. In a longitudinal study following 1157 children, we investigated presence of siblings, center care exposure in the first 3 years of life (attendance, hours, and child group size), and family socioeconomic status (SES) as predictors of hot and cool effortful control (EC), at the child's age 48 months. The results showed that having a sibling was consistently related to better hot EC, whereas higher SES predicted better cool EC. A small effect implied that hours in center care at 36 months negatively predicted hot EC, whereas center care group size at 36 months modestly predicted better cool EC. Otherwise, center care variables were unrelated to self-regulation. (author abstract)