Social trust (ST) (i.e., beliefs that people are generally fair and trustworthy) is a critical disposition for democratic governance. Yet there has been scant research on its developmental foundations. We assess factors related to ST in 11-18 year olds with survey data collected over two years from 1150 U.S. adolescents and their mothers. Adolescents' ST in year 1 and their reports of a positive neighborhood climate predicted ST one year later. Adolescents' reports of family practices were stronger predictors of their ST than were mothers' reports. Regression analyses revealed different factors predicting changes in ST for three adolescent age groups: With ST at T1 and background factors controlled, democratic parenting boosted ST for early- and middle-adolescents. Adolescents' reports that parents encouraged compassion for others boosted ST for middle- and late-adolescents, and parental cautions about other people taking advantage diminished ST among middle adolescents. Results suggest that the disposition to trust others is formed, in part, by what adolescents hear from parents about their responsibilities to fellow human beings and by modeling of democratic parenting. (Contains 4 tables.)