The plight of the middle class is one of the hottest topics in the U.S. presidential campaign. Candidates compete to offer plans for the revival of the dependable middle, the foundation of society. The polarization isn't only occurring in the U.S., though. In Germany, with its far more socialist income redistribution model, the middle class also is endangered.

In both countries, the middle class is shrinking as a share of the general population, and so is the share of income that accrues to it. But the reasons for the contraction tell a lot about the different priorities of the two societies.

Markus Grabka, Jan Goebel, Carsten Schroeder and Juergen Schupp explored the comparison in a paper published by the German Institute for Economic Research. They defined the middle class as those with income between 67 percent and 200 percent of the nation's median. (The U.S. median income was $54,629 and it was $47,773 in Germany, according to the latest data from the World Bank.)