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A standard optimal investment model can be used to analyse an individual's decision to accumulate social capital. We analyse six facts that support the predictions of this individualbased approach: (1) social capital first rises and then falls with age, (2) social capital declines with expected mobility, (3) social capital rises in occupations with greater returns to social skills, (4) social capital is higher among homeowners, (5) social connections fall sharply with physical distance, (6) people who invest in human capital also invest in social capital. We fail to find robust evidence that social capital investments fall with the value of time or that geographic/religious groups generate social capital complementarities.

This paper presents on overview of the economics that lies behind social interaction models and briefly discusses the empirical approaches to social interactions. We present a simple model with local interactions, similar to Glaeser, Sacerdote and Scheinkman (1996) but using a continuous action space and starting with optimizing behavior. We then extend the model to include both global and local interactions. We suggest and use a methodology for using variation of intra-city aggregates to identify the relative sizes of local and global interactions. We also present a model with endogenous location choice and use the predictions of that model to identify the sources of cross-city variance that are due to sorting and interaction. Finally, we present a brief discussion of using time-series to estimate the social interactions in broad aggregates.

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