The debate over inequality has focused mainly on taxes, fairness and government.
But two new academic papers shed light on one of the biggest drivers of wealth inequality—the higher investment returns of the wealthy. The growing wealth of the rich, and the relative stagnation of the middle class, are due in large part to diverging incomes, but investments also play an increasingly important role.
And it's not just that the wealthy have more investments. They also have better-performing investments.
Read MoreWidening gap: haves vs. the have-mores
In a paper on household wealth over the past decade, economist Edward Wolff at New York University found that wealth inequality rose sharply from 2007 to 2010 and has remained largely unchanged since then.