"The basic marriage rule in Hindu society is that no individual is permitted to marry outside the sub-caste or jati. Social mobility will be severely restricted by this rule because individuals are forced to match within a very narrow pool. Social mobility, as measured by inter-caste marriage, continues to be low in rural India despite the economic changes within and across castes that have taken place over the past decades. Recent surveys in rural and urban India that the authors have conducted indicate that among 25-40 year olds, out-marriage was 7.6% in Mumbai in 2001, 6.2% in South Indian tea plantations in 2003, and 5.8% for the rural Indian population in 16 major states of India in 1999."
This from a fascinating new NBER working paper by Munshi & Rosenzweig.