The assumption you're making that you haven't substantiated is that higher (as in socially-more-powerful) castes have higher IQs. (There are a number of other

The assumption you're making that you haven't substantiated is that higher (as in socially-more-powerful) castes have higher IQs. (There are a number of other unsubstantiated assumptions beyond that, including that the difference in IQ is enough to explain the observed effect, that the variance in the distribution of IQs in each population is pretty close to zero, that castes started out as physically-isolated populations with distinct IQs, etc., but let's start with that one.)