Chapter 2 · Competitive markets: Demand and supply
Allocative efficiency: MB = MC
See why MB = MC maximises total surplus. Compare marginal benefit and marginal cost, shade consumer and producer surplus, and explore producing too little or too much.
The economic idea
Marginal means additional. Marginal benefit is the extra benefit from consuming one more unit; marginal cost is the opportunity cost of producing one more unit. Compare these two values to ask whether that unit is worth producing.
Allocative efficiency occurs at the output where marginal benefit equals marginal cost: MB = MC. Below that output, an extra unit adds more benefit than cost. Beyond it, the extra cost exceeds the benefit. Total net benefit is therefore greatest at the efficient output in this model.
Demand represents marginal benefit, so “D = MB”: the height of the demand curve shows willingness to pay for the next unit. Supply represents marginal cost, so “S = MC”: its height shows the cost of the resources needed to produce the next unit.
Consumer surplus is willingness to pay minus the price paid, summed over purchases. Producer surplus is receipts minus the minimum producers would accept, summed over sales. In this model their sum is total surplus, also called social or community surplus. It measures net gains from exchange, not the amount of money changing hands.
Maximising total surplus does not require consumer surplus and producer surplus to be equal. What matters for allocative efficiency is their combined area, not which group has the larger share.
The output that maximises total surplus
“Quantity” is measured horizontally. The vertical axis, “Price, costs and benefits”, measures benefit or cost per unit, not total benefit or total cost.
“D = MB” slopes downward: successive units have lower marginal benefit. “S = MC” slopes upward: additional units require resources with a higher marginal opportunity cost.
Their intersection “E*” identifies “Q*”. The output control selects a quantity to compare with this benchmark; it does not shift either curve or impose a price control.
Start with the marginal comparison
“D = MB” and “S = MC” meet at “E*”. At “Q*”, marginal benefit equals marginal cost. All units whose benefit exceeds their resource cost are included, and output does not extend into units whose cost exceeds their benefit.
Read the areas as net benefits
At the competitive benchmark price “P*”, consumer surplus is below “D = MB” and above the price, up to “Q*”. Producer surplus is above “S = MC” and below the price over the same output. The two areas together form the maximum total surplus in this model.
Why equality at the margin does not mean zero surplus
At “Q*”, MB = MC applies to the marginal unit. Earlier units have a benefit greater than their cost. Adding those positive differences gives the area between the curves up to “Q*”; it is not erased when the curves meet.
The vertical distance between the curves is a gain or loss per additional unit. An area adds those differences over a range of output. Do not confuse a per-unit value with total welfare.
Connect consumer and producer surplus to the total
The price “P*” divides the total-surplus area between consumers and producers. The efficiency condition comes from “D = MB” meeting “S = MC”, not from the two shaded areas having equal sizes.
A payment from a buyer to a seller changes who receives money; by itself, it is not a resource loss to society. Lost gains from missing beneficial units, or excess costs from harmful extra units, are different from transfers.
Producer surplus is not automatically profit. It measures receipts above the minimum needed to supply the units shown; a firm’s profit also takes account of fixed costs that this marginal-cost diagram does not display.
Common mistakes to avoid
- Compare marginal benefit with marginal cost at the selected quantity. The point where MB = MC identifies the efficient output.
- The efficient output maximises total surplus, not consumer surplus alone, producer surplus alone or equality between the two.
- MB = MC does not mean total benefit equals total cost or that total surplus is zero.
- The output selector is a hypothetical welfare comparison. It does not show why a real market underproduces or overproduces; a question must supply that cause.
- Underproduction and overproduction describe output relative to the efficient quantity, not a market shortage or surplus at a posted price.
How to draw this diagram
- Label the horizontal axis Quantity and the vertical axis Marginal benefit, marginal cost, price.
- Draw downward-sloping demand labelled D = MB and upward-sloping supply labelled S = MC.
- Mark their intersection E* and project the efficient quantity Q* to the horizontal axis.
- At the competitive price P*, shade CS below demand and above price, and PS above supply and below price, up to Q*.
- For an output below or above Q*, mark the selected quantity. Shade the difference between MB and MC over the missing or excess units; explain why that area is forgone benefit or excess resource cost.
Assumptions and limitations
- This model uses willingness to pay to measure marginal benefit and the supply curve to measure marginal resource cost.
- The selected output is a hypothetical allocation. Outside the efficient benchmark, no price, policy, rationing arrangement or cause of the allocation is specified.
- Changing the selected quantity holds demand and supply unchanged. A shift in either curve is a different change and would move the efficient output.