> A recent study shows that the overall rate in both whites and blacks is about 15%, while in coloureds it is 40%.
Worth knowing that "colored" is a loaded term in American vernacular, and may provoke strongly negative responses depending on context - I know that's not how you're using it, but many would not, and might take it amiss therefore. "Black" is a denotative equivalent lacking the same connotations, and what I'd recommend using instead in discourse with predominantly American interlocutors.
To your point, I gather there's a class divide along racial lines in South Africa, just as (albeit more sharply defined than) in the US, and would wonder whether that's more where the correlation lies. I think it must be here - that's my observation, at least, anecdotal though it be.