Marginal Social Benefit vs Marginal Social Cost
Marginal Social Benefit and Marginal Social Cost are two Market Failure & Government concepts in AP Economics that students often mix up. Marginal social benefit is the total benefit to society from consuming one more unit, equal to private benefits plus external benefits. Marginal social cost is the total cost to society of producing one more unit, equal to private costs plus external costs. Here is how they compare side by side.
It equals the marginal private benefit plus the marginal external benefit. With a positive externality, the marginal social benefit curve lies above the demand curve. The socially efficient quantity occurs where marginal social benefit equals marginal social cost.
It equals the marginal private cost plus the marginal external cost. With a negative externality, the marginal social cost curve lies above the supply (marginal private cost) curve. The socially efficient quantity occurs where marginal social cost equals marginal social benefit.
Marginal Social Benefit vs Marginal Social Cost: Which Curve Carries the Spillover
| Marginal Social Benefit | Marginal Social Cost | |
|---|---|---|
| Which curve you build it from | The demand curve, raised when consumption spills a benefit onto outsiders and lowered when it spills a cost | The supply curve, raised when production spills a cost onto outsiders and lowered when it spills a benefit |
| Slope on the standard graph | Downward sloping, since the next unit is worth less than the last | Upward sloping, since the next unit costs more than the last |
| Where the third party is standing | Outside the transaction but touched by the consuming, like a neighbor protected by someone else's vaccination | Outside the transaction but touched by the producing, like a fishery below a plant's outfall |
| What goes wrong and what fixes it | In the usual case too few units, corrected by a per-unit subsidy equal to the marginal external benefit | In the usual case too many units, corrected by a per-unit tax equal to the marginal external cost |
| Which edge of the loss triangle it forms | The top edge when output is too low, since the missing units are worth more than they cost | The top edge when output is too high, since the extra units cost more than they are worth |
In most problems only one of the two curves carries the spillover
The first move on any externality question is deciding which side the spillover sits on. A factory whose production dumps costs on people downstream creates a wedge on the cost side, so marginal social cost lies above marginal private cost while marginal social benefit sits exactly on the demand curve. A vaccination whose protection reaches people who never bought a dose creates a wedge on the benefit side, so marginal social benefit lies above demand while marginal social cost sits on the supply curve. Shifting both curves is the most common way to lose the point, because it usually leaves the quantity roughly unchanged and hides the whole result. Direction deserves a check too, since the social curve does not always sit above the private one. Bees kept beside an orchard raise the neighbor's yield, a benefit that comes from the act of producing, which puts marginal social cost below marginal private cost. A good whose consumption harms bystanders runs the same way in reverse and puts marginal social benefit below demand. Ask where the third party stands and whether the spillover helps or hurts, then split one curve, once.
A worked overproduction case, with the loss in dollars
Let demand be P equals 60 minus Q, and let marginal private cost be P equals Q. The market clears where 60 minus Q equals Q, so quantity is 30 and price is $30. Now suppose each unit produced imposes $6 of cost on third parties. Marginal social cost becomes Q plus 6, and efficiency requires 60 minus Q to equal Q plus 6, which gives a quantity of 27. The market makes three units too many. Each of those three costs society more than it is worth, and the vertical gap between marginal social cost and marginal social benefit grows from zero at 27 units to $6 at 30 units. Deadweight loss is the triangle with base 3 and height $6, so $9. A per-unit tax of $6 lifts marginal private cost onto marginal social cost, moves output to 27 and erases the loss, which is why the corrective rate matches the external cost rather than the quantity error.
Which curve sits on top at the market quantity decides tax or subsidy
At the quantity the market actually produces, compare the two social curves. If marginal social cost is above marginal social benefit, the last units cost society more than they deliver, output is too high, and the fix is a per-unit tax. If marginal social benefit is above marginal social cost, society wants units nobody is making, output is too low, and the fix is a per-unit subsidy. The size of the correction is the vertical gap between the private and social curve, never the horizontal gap between the two quantities. In the worked case above the vertical distance is $6 per unit while the quantity error is three units. Mixing the two gives you either a tax of $3, borrowed from the quantity gap, or a loss of $18, which is the rectangle rather than the triangle. Set the per-unit instrument equal to the external effect per unit, then let quantity adjust on its own.
Frequently asked questions
Where do marginal social benefit and marginal social cost intersect?
Marginal social benefit and marginal social cost cross at the socially efficient quantity, the output level a corrective policy is aiming for. To the left of that point society values the next unit more than it costs to make, so the unit should be made. To the right, the unit costs more than it is worth. A free market lands on that intersection only when there are no externalities, because the market equates private benefit with private cost instead.
Is marginal social benefit the same as the demand curve?
Marginal social benefit lies on the demand curve only when consumption creates no spillover for third parties. Demand traces marginal private benefit, the value buyers capture for themselves. When a purchase also helps people who did not buy it, marginal social benefit sits above demand by the marginal external benefit, and that vertical distance is what a corrective subsidy should equal. For a good with a production externality and no consumption externality, the two curves coincide.
How do I find deadweight loss using these two curves?
Deadweight loss is the triangle between the two curves across the units the market gets wrong. Locate the efficient quantity where marginal social benefit meets marginal social cost, then locate the quantity the market actually produces. The base of the triangle is the difference between those quantities and the height is the vertical gap between the curves at the market quantity. With a three unit error and a $6 gap, the loss is one half times 3 times $6, which is $9.
Live Externalities graph. Drag the curves, or open the full version.
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