I see these assumptions in Israel alot, specially when it comes to what the arabic people think about the jewish population and vice versa. Except, that (in my opinion) the jewish population assumes so much more about the arabic population and the cities they live in. Most of it comes from the media and the education at schools (we have different education systems for arabic schools vs. jewish schools). I had to learn this the hard way when I first encountered the israeli Burning Man community, I have overcome most of the first problems by actually realizing that I am the one who made most of the wrong assumptions even though I thought that I am not doing them,,,I'm not sure we can turn it off, but we can monitor it. We can note our assumptions and look for further evidence before "believing" them. Also, we have to avoid becoming too fond of our judgements, assumptions, and perceptions.
One technique is to train yourself to throw away the first interpretation of an event, observation, or behaviour and look for another, less obvious one. Then look for a third.