There is quite a lot of correlation between economic inequality and conflict. See the marvelous recent book The Spirit Level: Why More Equal Societies Almost Always do Better by Richard Wilkinson (whose work I have admired for over 30 years) and Kate Pickett. This is Richard and Kate photo below. I will post on this another time as it has huge consequences that have been taken up by all three major political parties in the UK.
In the meantime, economists measures of inequality include the GINI index under which if everyone had the same income the index would be 0 and if Bill Gates or whoever had everything it would be one. Here is a map of the world with the relevant 2009 data as supplied by the CIA, perhaps unjustly not known for its interests in such matters.