This paper offers new findings which support the hypothesis that a causal link from happiness to social capital might exist. The paper exploits the very long German socio-economic panel of around 15000 people. Using the prospective study methodology, it finds that happier people contribute more to social capital. Both parametric and nonparametric results suggest that there exists an inverted-U shape relationship between happiness to social capital. Moreover, optimism appears to be an important channel through which happiness is linked to social capital. The paper also presents residual happiness as a measure of optimism which might be a valuable tool for empirical researchers. The results are robust to inclusion of various controls including the initial level of social capital, random sampling, non-linearity, different measures of social capital, and estimation techniques.