I am indebted to my advisor Juergen Eichberger for his helpful guidance and to Alexander Zimper for his helpful suggestions and illuminating discussions. I would like to thank Hans Haller, Clemens Puppe, Itzhak Gilboa, Hans Gersbach, Klaus Ritzberger, as well as the participants of the RUD conference in Evanston, of the FUR XI conference in Paris and of the PhD seminar at the University of Heid...