To clarify, I'm not looking in this article at 'white nations', 'arab nations' etc. I'm looking at 'diverse nations' vs. 'homogenous nations'. For reference, the United States has an 80% ethnic majority (white).
I wanted to see if there was any difference in the diversity level of countries in each rating level. One way to do this would be to show histograms for each of the different ratings, but overlaying 7 histograms is pretty messy, and putting them side by side is hard to compare. Instead I'm showing cumulative proportions for multiple lines. The way to interpret this is that for each point, y% of nations in that rating have x% or less largest ethnic share.
I had expected that countries with the worst human rights abuses would have moderate ethnic majorities, and that was more more or less shown in the data. It turns out that the countries with the best human rights records also had clear ethnic majorities. In fact, the worst countries and the best countries are more similar in terms of the distribution of largest ethnic share than they are to the countries in the middle of the pack!
I suspect that this is a sort of convergence. A lot of the countries with the best human rights records are wealthy industrialized nations where most minorities are immigrants. Many of the countries with poor human rights records are developing nations where one indigenous group gained dominance through conquest, and there is a long history of ethnic conflict.
One word of caution - there aren't that many data points here, the worst example being civil rights ranking=7, with only 7 data points.