We document a causal effect of social interactions on investor behavior using the number of local

soccer games as a measure of social interaction intensity. Social transmission is identifiable in buy

but not sell trades. Social Interaction Intensity (SII) increases the sensitivity of buying to past buys,

particularly in riskier stocks. This sensitivity is an increasing and convex function of past returns,

with higher SII further amplifying the effect. Social interactions cause an extremity shift wherein

existing shareholders increase their positions, especially within demographically homogeneous

communities. Higher social interaction intensity increases the sensitivity of individual investors’

trading volume and portfolio riskiness to past trades. At the market level, SII increases the

sensitivity of stock trading volume and retail ownership percentage to past buys.