Social, or ’generalized‘, trust refers to beliefs that people hold about how other people in society will in general act towards them. Can people in general be trusted? Or must one be careful in dealing with people? Research on the antecedents of social trust has typically relied on cross-sectional regression estimators to evaluate putative causes. Our contention is that much of this research over-estimates the importance of many of these causes because of the failure to account for unmeasured confounding influences. In this paper we use longitudinal data to assess the causal status of a particularly prominent mooted cause of trust: the degree to which individuals are socially integrated via formal membership of civic organisations and through friendship networks. We fit a range of regression estimators to repeated measures data from the UK for the period 1998 to 2008. Our results show little support for the widely held view that social trust results from integration within social networks, of either a formal or an informal nature.