I should point out that the conclusion of the linked study is complete bullshit. It claimed that poor people are more polite than rich people. What it should have claimed was that poor people who went to Berkeley are more polite than rich people who went to Berkeley.

Poor people who go to Berkeley are likely to be moving up in the world relative to their families; their eyes are being opened to a whole new world of knowledge and power and all that, and they are humbled by it.

Rich people who go to Berkeley are likely to come from prep schools, and feel vindicated by having been admitted to a top school, and therefore are snooty.

The conclusion doesn't jibe at all with my personal experience, either, and I think this is why.