It seems fairly believable that an oppressed underclass that is intentionally deprived of education and opportunity will, on average, be cruder, less intellectually inclined, have less wealth and status, and more prone to failing at life in various ways due to the lack of a support structure. This is true of any group, whatever their intrinsic nature, simply due to the act of discrimination.

I remember once reading an essay about Jews in(IIRC) Rudyard Kipling's works, where they're portrayed in pretty appalling ways, while all sorts of other groups are portrayed positively. The author came to the conclusion that acting in cowardly and profiteering fashion was a survival tactic created by anti-semitic laws, and that Kipling was probably just conveying the reality of the time. (I'm not enough of an expert to judge the truth of this, but it seemed reasonable)