For a short and clear discussion on income inequality, read this article by professor Robert H. Frank:
Income Inequality: Too Big to Ignore
"Recent research on psychological well-being has taught us that beyond a certain point, across-the-board spending increases often do little more than raise the bar for what is considered enough. A C.E.O. may think he needs a 30,000-square-foot mansion, for example, just because each of his peers has one. Although they might all be just as happy in more modest dwellings, few would be willing to downsize on their own.
People do not exist in a social vacuum. Community norms define clear expectations about what people should spend on interview suits and birthday parties. Rising inequality has thus spawned a multitude of “expenditure cascades,” whose first step is increased spending by top earners."
"The middle-class squeeze has also reduced voters’ willingness to support even basic public services. Rich and poor alike endure crumbling roads, weak bridges, an unreliable rail system, and cargo containers that enter our ports without scrutiny. And many Americans live in the shadow of poorly maintained dams that could collapse at any moment.
ECONOMISTS who say we should relegate questions about inequality to philosophers often advocate policies, like tax cuts for the wealthy, that increase inequality substantially. That greater inequality causes real harm is beyond doubt.
But are there offsetting benefits?
There is no persuasive evidence that greater inequality bolsters economic growth or enhances anyone’s well-being."