I don't know. Just as one example, I've seen here on HN ads for positions that explicitly mention (in not so many words) that they're only looking for grads from elite colleges. The UK just codified this into visa law.

Income is a fuzzy thing, and distinct from opportunities.

I'm always looking for empirical data to bear on a question, so the studies you're mentioning are important. But at the same time I'm sort of suspicious that something's missing from those studies based on prima facie examples I've encountered several times in my life.