I think it's part of the genome. Nature meant for us (or more specifically, random processes of evolution/natural selection produced the outcome) to have a mixture of, at its most basic level, "competitors" (generally them) and "cooperators" (generally us) because it increases overall survival chances for the species. We both need to exist to curb the other's tendencies and balance things out.
A good analog can be found by studying the social structures of our two closest cousins, Chimps and Bonobos. Their side is more "chimp-like", and ours is more "bonobo-like". But both their societies I'm sure if it were studied would find there are bonobo-like chimps, and chimp-like bonobos if you drill down to individuals.
If you put what I just said into AI and ask it to analyze whether what I just said tracks with available facts, it should give you an interesting answer. Tell it by 'us' and 'them' I mean liberals and conservatives, and generally what it's regarding (e.g. the studies mentioned in your OP) 
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