Numerous scholars have shown that increasing ethnic diversity is correlated with reduced cooperation and fewer public goods. This result has significant implications for development policy, lowering expectations for success in public infrastructure investment with ethnically diverse populations. I present evidence that ethnic hierarchy may be driving the observed effect of ethnic diversity. Ethnic hierarchy is naturally confounded with ethnic diversity because hierarchy cannot logically exist without diversity and because diversity without hierarchy may be exceedingly rare. To determine which factor is a greater constraint on cooperation, I tested the strength of both ethnic diversity and ethnic hierarchy as cooperative limitations using public goods experiments with caste groups in South India. I show that the effect of ethnic diversity is neutralized when relatedness between individuals is taken into account. However, ethnic hierarchy remains severely damaging to public goods cooperation, when all variables are accounted for, and is a required variable for any explanatory model. Moreover, the influence of social momentum is significant. I find that the initial behavior in the game determines the long run outcome, while the cooperative momentum is carried forward in the round-by-round decisions.