The two go hand in hand, think about it. There is always an elite in every society, and there is always the impoverished. The middle class is a buffer that enables political stability. When the ranks of the poor grow, it's always because the middle class is becoming poorer. There are no historical instances of a growing disparity in wealth where the middle class shrank, the wealthy grew, and the poor remained the same. The middle class always shrinks towards poverty.
Not necessarily, I am looking for info on JUST income disparity.