Social and factor graphs require the correct marginal curves

Private versus social values and wage versus marginal revenue product answer different questions.

For a negative production externality, market supply is MPC and social cost MSC lies above it. Market quantity occurs where demand meets MPC. Efficient quantity where MSB meets MSC. For a positive consumption externality, MSB lies above private demand.

Rebuild a graph from one sentence

“A binding ceiling is imposed below equilibrium.” Draw supply and demand, mark their intersection, and place the ceiling below it. At that legal price, read quantity supplied from supply and quantity demanded from demand. The horizontal distance is the shortage. This construction is safer than trying to recall a finished picture because every feature follows from the sentence.

Describe the graph in one sentence

After drawing, say: “The event shifts ___ because ___, so equilibrium ___ and ___.” If you cannot complete the mechanism, the picture is not yet reliable.

Advanced-looking diagrams become manageable when every curve is translated into a marginal sentence. An externality graph asks whether private buyers and sellers face all costs and benefits. A factor graph asks how much revenue another input creates and what hiring it costs. The drawing follows those meanings.

For negative production externalities, ordinary supply is marginal private cost and MSC lies above it by marginal external cost. The market chooses where demand or MSB meets MPC. Society prefers where MSB meets MSC. For positive consumption spillovers, ordinary demand is MPB and MSB lies above it. The direction of inefficiency follows from the omitted value, not from a memorized curve color.

Place the missing curve

A factory’s next unit costs it $14 and causes $6 of external damage. At that quantity, MSC is $20. If buyers’ marginal benefit is $17, the unit is privately worthwhile but socially wasteful. The social-cost curve must lie above private supply, and efficient output is lower than market output.

In a competitive labor market, downward labor demand represents MRP and upward labor supply represents workers’ opportunity cost. Their intersection determines wage and employment. One competitive firm takes the wage as given and hires until its MRP equals that wage. A change in product demand can shift labor demand because it changes the revenue created by workers.

The curves should not be mixed. A competitive wage comes from market labor supply and demand. Private supply is not MSC with pollution. Market demand is not MSB when consumer benefits spill over. A correct label can be more important than the curve’s artistic shape.

Use a five-part audit: identify the chooser, label the vertical unit, state what each marginal curve includes, find the decision intersection, and then locate any price or wage on the appropriate nonmarginal curve. This method handles unfamiliar graphs because it rebuilds them from incentives rather than recalling a snapshot.

Six-graph closed-book drill

On six blank sets of axes, rebuild: market supply and demand after one shift. A per-unit tax wedge. A competitive firm with profit or loss. A single-price monopoly. A negative externality. And a competitive labor market. For each drawing, label both axes, every curve, the original decision, the changed decision, and one sentence explaining the mechanism. Check the book only after all six are complete.