Video - must watch (Wilkinson begins at about 5:00).
I subscribe to a sort of evolutionary adaptive environment/laws written in the hearts of humankind view of this. It is greed and hard-heartedness that is the fundamental problem with America and other highly unequal countries. The really interesting thing is that the harm of inequality is spread among all income and status classes, and doesn't just affect those at the bottom.
The Equality Trust has summaries of the research.