Well that's kind of my point. All humans use their preconceived beliefs to interpret data. That's just part of what we do, it makes sense to try to fit what you see in to what you know. However parties that are built differently are naturally going to react differently to conflicting data, and furthermore as you say, they're not heterogeneous bodies, they're fairly diverse.

You think the two party system doesn't have an effect on politics? That's an odd thing to say. But I'm not saying that people will not be swayed by their preconceived ideologies, I'm saying that its easier to realize they're not just two sides of a coin.