Ecological economists believe that economic possibilities are constrained by natural laws (for example, the laws of thermodynamics, biological assimilation, and the limiting factor). Unlimited material growth from a finite resource base—spaceship earth—is therefore impossible. How many “earths” would it take for everyone to live like U.S. citizens? (To help answer this question, you might visit and take the test at http://ecological/footprint.) (Institutionalist)