Journal article: This paper uses the variations of ethnic diversity between districts in Indonesia to show that diversity leads to lower social capital outcomes. I find that distinguishing between ethnic polarization and fractionalization matters for the results, as polarization has a larger negative effect. The results cannot entirely be attributed to selection on unobservables, and at least part of the relationship should be interpreted as causal. Finally, diversity seems to increase tolerance, despite its negative effect on other social capital variables such as trust, perceived safety, and participation to community activities, and voting in elections.