Given the importance that generalized social trust plays in various theories of American society, recent evidence of its low levels among younger people portends ominous changes in American civic life. Using survey data collected from high school seniors over the last 20 years, this paper examines the origins of social trust among young people and the causes of change in beliefs about trust over time. Such changes could not be accounted for by the explanations for declining trust offered in other accounts of social capital. An alternative explanation, based on the theoretical accounts of Alexis de Tocqueville and Emile Durkheim, is that materialistic values may be undermining young people's views about the trustworthiness of others. Both aggregate time series correlations and an individual-level model show that the rapid rise of materialistic value orientations that occurred among American youth in the 1970s and 1980s severely eroded levels of social trust. The paper concludes with some observations about the likely trajectory of American democracy, given the kinds of trends observed in the youth data.