Social, or 'generalized' trust is often characterised as the 'attitudinal dimension' of social capital. It has been posited as key to a host of normatively desirable outcomes at the societal and individual levels. Yet the origins of individual variation in trust remain something of a mystery and continue to be a source of dissensus amongst researchers across and within academic disciplines. In this paper we use data from two British birth cohort studies to test the hypothesis that a propensity to express generalized trust varies systematically as a function of individual intelligence. Intelligence, we argue, fosters greater trust in one's fellow citizens because more intelligent individuals are more accurate in their assessments of the trustworthiness of others. This means that, over the life-course, their trust is less often betrayed and they are able to accrue the benefits of norms of reciprocity. Our results show that standard measures of intelligence administered when cohort members were aged 10 and 11 can explain variability in expressed trust in early middle age. net of a broad range of theoretically related covariates. (c) 2009 Elsevier Inc. All rights reserved.