income

Here's an interesting picture (taken from http://rodrik.typepad.com/dani_rodriks_weblog/2008/03/american-politi.html). Basically, under Republican presidents income disparity has a tendency to increase, and vice versa for Democratic presidents. And income increases more overall under Democratic presidents.

The book which the article references apparently goes to great lengths to demonstrate why this difference isn't a fluke. Pretty interesting (written by a Princeton professor, too).