IQ and many personality traits have been shown to be correlated significantly with genes (many of these studies are based on adopted children, for whom genes and upbringing are decoupled). In monogamous societies like in America, someone who's highly smart and industrious, will likely end up with the same number of children as someone on the opposite end of the spectrum. In polygamous societies, someone who's smart and industrious can easily end up with 2-10x the number of children as the average man. In terms of natural selection, this clearly puts significantly more selective pressure on intelligence and personality-traits.
Over a few decades, I doubt this will amount to much. But over a couple centuries/millenia, I wonder if polygamous societies would wind up as a race of smart-hard-working-competitive-assholes, in comparison to us monogamous dolts.