Gil Skillman:
Well, the most general answer is that an " economic rent" arises anytime someone receives a return greater than the amount necessary to induce them to stay in the activity that yields the return. The question is what makes this possible. As your own examples suggest, "scarcity" of some sort or other is necessary, but not sufficient; you also need *some* form of unequal distribution, if only unequal distribution of "psychic opportunity costs" (yielding differential supply prices, so that inframarginal suppliers earn rents even if marginal suppliers don't). Otherwise, you could have "scarcity" in the neoclassical sense--i.e., opportunity cost--but no economic rents.
a rent is received because of some sort of _special_ advantage that someone has (almost by definition, if not exactly so). That means that there must be some sort of distributional content to it. If the advange is distributed evenly among everyone, there is no special advantage and no rent. (It's a _general_ advantage.) -- Jim Devine / "Mathematicians are like Frenchmen: whatever you say to them, they translate it into their own language, and forthwith it means something entirely different." -- Johann Wolfgang von Goethe
