Table of Contents

Perloff 10; WB 10 and 11.
The first model we study resolves a problem with the supplyanddemand model that we ignored earlier. There, the price P^{G} of any related good is assumed to be exogenous — not affected by what happens in the market for this good. However, we know that price changes are reinforced by feedback effects. As a result, no matter whether the two goods are complements or substitutes, our supplyanddemand analysis understates their crossprice responses.
To solve this problem, we need all prices to be endogenous — variables determined in the equilibrium. In general equilibrium models, “prices” represent the terms of trade, not monetary transfers. To fully solve the feedback problem, the model must include markets in everything, or complete markets.
The second general equilibrium model is set at the level of individual decisions, instead of Demand and Supply curves. It illustrates the key economic concept of efficiency and the related Welfare Theorems. We can use the Consumer Choice model to understand these decisions, replacing “income” with how much of a particular good each individual can get by trading the goods they have.
Two markets
Solving the feedback problem mentioned above, we consider the markets for two related goods at once. See Perloff, Ch 10, Question 10 and Problems 24, 25.
 Given formulas for the quantities supplied and demanded in two markets, find the equilibrium prices and quantities of each good.
Pure exchange
No one produces goods in this model.^{1} Two individuals A and B start out with different endowments of the goods X and Y, and choose how much to trade with each other.
The endowment is the initial allocation — distribution of goods — and every point in the Edgeworth box is a different allocation (X_{A},Y_{A},X_{B},Y_{B}) that can be reached by trade.
 Given endowments (X_{A}^{0},Y_{A}^{0},X_{B}^{0},Y_{B}^{0}), draw the Edgeworth box, identifying its size and the endowment allocation.
 Given endowments (X_{A}^{0},Y_{A}^{0},X_{B}^{0},Y_{B}^{0}) and CobbDouglas preferences, find the equilibrium allocation (X_{A}^{*},Y_{A}^{*},X_{B}^{*},Y_{B}^{*}) and prices (with one of the prices fixed at 1).
Efficiency
A Pareto improvement makes at least one individual better off and none worse off. A situation is (Pareto) efficient if no Pareto improvement can be made. This definition of efficiency can be applied beyond the pure exchange model.
 With CobbDouglas preferences, what do we know about the individuals' MRS's at any efficient allocation?
 Given an inefficient situation, propose a trade (or other change) that leads to a Pareto improvement.
Given an an endowment allocation (X_{A}^{0},Y_{A}^{0},X_{B}^{0},Y_{B}^{0}) and an Edgeworth box with ICs
 Place the endowment allocation.
 Identify the set of efficient allocations (the contract curve).
 Identify the set of allocations that are Pareto improvements over the endowment.
 Identify the set of efficient allocations that are also Pareto improvements over the endowment (the core).
The First and Second Welfare Theorems clarify the connection between equilibrium and efficiency.
 Are all efficient allocations also equilibrium allocations? Are all equilibrium allocations efficient?
More resources
 Perloff, Chapter 10 ``General Equilibrium and Economic Welfare''
 Quizzes
 Applications
 Stiglitz and Walsh, Chapter 10 ``The Efficiency of Competitive Markets''
 Narrated lecture with graphs
 Quizzes
 FAQs and pitfalls
 ESL's Edgeworth Box (Java applet)
More information
Arbitrage (related to efficiency…) of gasoline