[Apologies if this repeats an already-posted reply.] 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.
what are the main types of "rent" in economics? (I'm not talking about a user fee, as paid to rent an apartment.) there's a natural-scarcity rent (Ricardo), technical rent (superior technology), what's now called monopoly rent (monopoly profits), and politically-granted rent (super-profits due to tariff protection and the like). Any others? -- 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
