Wealth and Democracy: A Political History of the American Rich http://www.amazon.com/Wealth-Democracy-Political-History-Ame...
Gross inequity is undemocratic. I cite Kevin Phillips' book Wealth and Democracy http://www.amazon.com/Wealth-Democracy-Political-History-Ame... but it's been researched and detailed ad nauseum.
More equitable societies are happier.
More equitable societies have better public health.
Just in case you need cites: https://www.google.com/searchq=More+equitable+societies+are+...
Lastly, I believe, but cannot prove, that more equitable societies are safer. Both domestically and internationally.
Inequity is the result of math. The rich get richer. Without progressive income redistribution, increasing inequity is inevitable.
This is why game shows have bonus rounds, mooting the first 1/2, so that people continue watching.
Further, creation of the middle class is a deliberate, explicit policy decision. Not the natural outcome of Freedom Markets (tm). The USA has previously chosen to nurture a middle class on three separate occasions. We're overdue for a 4th restructuring, say by 20 years.
Wealth and Democracy: A Political History of the American Rich
http://www.amazon.com/Wealth-Democracy-Political-History-Ame...