India's Caste Endogamy Was Very Extreme

The interracial intermarriage rate in Jim Crow America was about 1%. At that rate, if introgression of non-white individuals into whites continued for 50 years, the average person with white ancestry would be 40% non-white. (I suspect this exaggerated figure is due to improper use of a 1% introgression rate per year rather than per generation, but even a few generations of modest introgression does have a notable effect over a very long time period)

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In India, caste cemented itself, not just at the Varna level, but at the Jati level about 1500 years ago. This implied an endogamy rate on the order of 99.8% (with 10% introgression over that time period) to 99.9% (with 5% introgression over that time period) to 99.98% (with 1% introgression over that time period).

I think the source is Razib or someone he quoted, but I could be mistaken. I accidentally failed to hit the publish button on this post back on April 17, 2018.