In modern times it was a given that there was a large middle and smaller extremes. The middle class was the norm, with the very rich and very poor at the fringes- smaller in number.
The theory of the well curve assumes that the middle has been slowly disappearing, and the extremes are on the rise. This is hard to argue with when you consider the evidence in education (achievers and dropouts), economics (rich and poor), technology (small and big), politics (liberal and conservative). There are examples in almost every aspect of life.