What “socially efficient” means
Unit 6 asks a big question: when do markets give society the best possible use of its resources, and when do they fail? To answer it you need a clear benchmark.
For every unit of a good, society gains a benefit and pays a cost:
- Marginal social benefit (MSB) is the extra benefit to everyone in society from one more unit. It includes the buyer’s benefit and any benefit to other people.
- Marginal social cost (MSC) is the extra cost to everyone of producing one more unit. It includes the seller’s cost and any cost to other people.
A unit is worth making if its MSB is greater than its MSC: society gains more than it gives up. So the best quantity is found by making every unit with MSB > MSC and stopping where the two are equal.
The socially efficient (or socially optimal) quantity is where MSB = MSC for the last unit produced. At that quantity total economic surplus (total benefit minus total cost, or consumer surplus plus producer surplus, plus any benefits to others and minus any costs to others) is at its maximum. This is also called allocative efficiency: resources go to the goods society values most.
Notice that efficiency is about the size of total surplus, not how it is shared. A market can be efficient and still leave some people much better off than others.
Why a perfectly competitive market gets it right
In Topic 2.6 you saw that a competitive market’s equilibrium maximises total surplus. Here is why, in Unit 6 language.
- The demand curve shows buyers’ marginal private benefit (MPB): the most they will pay for each extra unit.
- The supply curve shows sellers’ marginal private cost (MPC): under perfect competition, market supply is the sum of the firms’ marginal cost curves (above average variable cost).
If nobody outside the market is affected, private and social values are the same: MPB = MSB and MPC = MSC. Economists say that all the benefits and costs are internalised: the people making the choice bear them all.
In a perfectly competitive market every buyer buys until MPB = P, and every firm produces until P = MC. So at equilibrium:
MSB = MPB = P = MPC = MSC
The market quantity is the efficient quantity. No single buyer or seller planned this. Each one followed private incentives, and competition made those incentives line up with society’s.
In Halvern, a fictional region, the monthly market for olive oil has no effects on outsiders. Q is in thousand litres per month and P is in dollars per litre:
- MSB = 50 − 0.5Q (this is the demand curve)
- MSC = 10 + 0.5Q (this is the supply curve)
Setting them equal: 50 − 0.5Q = 10 + 0.5Q, so Q* = 40 thousand litres and P = $30 per litre.
Total surplus = ½ × 40 × (50 − 10) = $800 thousand per month. Of this, consumer surplus is ½ × 40 × (50 − 30) = $400 thousand and producer surplus is ½ × 40 × (30 − 10) = $400 thousand.
How private incentives lead to inefficient outcomes
The good result above needs two conditions: competition and all benefits and costs internalised. Remove either one and rational people, acting in their own interest, produce the wrong quantity.
1. Market power. A firm with market power can raise its price by selling less. It is rational for a monopoly to produce where MR = MC and charge a price above MC (Topic 4.2). At that quantity the next unit is worth more to buyers (P) than it costs to make (MC), but the firm does not make it, because doing so would lower the price on all its other units. Output is too low.
2. Private and social values differ. Buyers and sellers make choices by equating private marginal benefit with private marginal cost. If some of the cost or benefit falls on other people, the market ignores it. Then the market quantity is not where MSB = MSC.
| Source of inefficiency | What goes wrong | Market quantity compared with the efficient quantity |
|---|---|---|
| Monopoly, oligopoly, monopolistic competition | price is above marginal cost (P > MC) | too little |
| Negative externality (e.g. pollution) | sellers or buyers ignore a cost to others, so MSC > MPC | too much |
| Positive externality (e.g. education) | buyers or sellers ignore a benefit to others, so MSB > MPB | too little |
| Asymmetric information | one side knows more than the other, so some trades worth making do not happen, or poor ones do | usually too little |
| Public goods | people can enjoy the good without paying, so they free ride and firms cannot sell it | too little, often zero |
You will study externalities in Topic 6.2, public goods in Topic 6.3 and policy in imperfect markets in Topic 6.4. In Topic 5.4 you also saw a monopsony hire fewer workers than a competitive labour market would: another case of market power cutting output below the efficient level.
Asymmetric information needs a short example. Suppose sellers of second-hand phones know which phones have hidden faults, but buyers cannot tell. Buyers will only offer a price that allows for the risk of a faulty phone. Owners of good phones then find the price too low and do not sell. Trades that would benefit both sides never happen, so the quantity traded is below the efficient level.
Deadweight loss
Deadweight loss (DWL) is the total surplus lost when the quantity is not the efficient one. It is surplus that nobody receives.
- Underproduction (Q below Q*): units with MSB > MSC are not made. DWL = the gain those units would have created.
- Overproduction (Q above Q*): units with MSC > MSB are made. DWL = the net cost those units impose.
With straight-line curves, the DWL is a triangle:
DWL = ½ × (gap between Q and Q*) × (gap between MSB and MSC at Q)
The triangle always points towards the efficient point, where MSB and MSC meet.
Fixing inefficiency: designing policy with cost-benefit analysis
If a market produces the wrong quantity, a policy can try to move it to the quantity where MSB = MSC. Examples you will meet: taxes and subsidies (Topic 6.2), public provision (Topic 6.3) and price regulation of a monopoly (Topic 6.4).
But every policy has its own costs: staff to run it, firms’ time to comply, and the risk of getting the numbers wrong. Policymakers use cost-benefit analysis: they compare the gain in total surplus (the DWL removed) with the cost of the policy. A policy is worth it only if the gain is larger. When several policies are possible, the best one is the one with the largest net gain, which is not always the one that removes all of the DWL.
Worked example 1: bookcases in Wrenford
Question. Wrenford is a fictional town. Its workshops make custom bookcases. No one outside the market is affected. The table shows the MSB and MSC of each bookcase made per week.
| Bookcase | 1st | 2nd | 3rd | 4th | 5th | 6th | 7th | 8th |
|---|---|---|---|---|---|---|---|---|
| MSB ($) | 300 | 270 | 240 | 210 | 180 | 150 | 120 | 90 |
| MSC ($) | 100 | 120 | 140 | 160 | 180 | 200 | 220 | 240 |
(a) Find the socially efficient quantity and the maximum total surplus. (b) Only one workshop survives and it sells 3 bookcases a week. Find the DWL. (c) Instead, a town grant leads workshops to make 7 bookcases a week. Find the DWL. (d) Explain why the single workshop in (b) is acting rationally.
(a) Find the net gain, MSB − MSC, for each bookcase:
| Bookcase | 1st | 2nd | 3rd | 4th | 5th | 6th | 7th | 8th |
|---|---|---|---|---|---|---|---|---|
| MSB − MSC ($) | 200 | 150 | 100 | 50 | 0 | −50 | −100 | −150 |
The gain is positive up to the 4th bookcase and zero for the 5th, where MSB = MSC = $180. After that it is negative. The efficient quantity is 5 bookcases per week. (Making the 5th adds nothing and loses nothing; by convention we produce up to MSB = MSC.)
Maximum total surplus = 200 + 150 + 100 + 50 + 0 = $500 per week.
(b) At 3 bookcases, the 4th and 5th are not made. Lost gains = 50 + 0 = $50 per week. Check: surplus at 3 = 200 + 150 + 100 = $450, and 500 − 450 = 50. ✓
(c) At 7 bookcases, the 6th and 7th are made even though they cost more than they are worth. DWL = 50 + 100 = $150 per week. Check: surplus at 7 = 500 − 50 − 100 = $350. ✓
(d) The workshop sells 3 bookcases at $240 each, the most buyers will pay for the 3rd. The 3rd bookcase costs only $140 to make, so P > MC by $100. To sell a 4th it must cut the price to $210 on all four. Revenue rises from 3 × $240 = $720 to 4 × $210 = $840, an extra $120. That is less than the $160 cost of the 4th bookcase, so profit would fall. (The 3rd was worth selling: it added 720 − 2 × 270 = $180 of revenue for $140 of cost.) So the workshop stops at 3. Its choice is rational for the workshop but inefficient for society.
Interpretation. Too little and too much both lose surplus. Here overproducing by 2 loses more than underproducing by 2, because the gaps between MSB and MSC grow the further you move from the efficient quantity.
Worked example 2: olive oil in Halvern, with a policy test
Question. Use Halvern’s olive oil market: MSB = 50 − 0.5Q and MSC = 10 + 0.5Q. (a) A cooperative gains control of every olive press and cuts output to 30 thousand litres. Find the price and the DWL. (b) In a different year, a grant to growers pushes output to 48 thousand litres. Find the DWL. (c) The government could regulate the cooperative so that it produces 40 thousand litres. Running the regulator costs $35 thousand per month. Is the policy worth it? What if it cost $60 thousand?
(a)
- Price is read from the demand (MSB) curve: P = 50 − 0.5 × 30 = $35 per litre.
- MSC of the 30th thousand litres = 10 + 0.5 × 30 = $25. So P > MC by $10.
- DWL = ½ × (40 − 30) × (35 − 25) = ½ × 10 × 10 = $50 thousand per month.
(b)
- At Q = 48: MSB = 50 − 24 = $26 and MSC = 10 + 24 = $34.
- MSC > MSB, so these extra units are not worth their cost.
- DWL = ½ × (48 − 40) × (34 − 26) = ½ × 8 × 8 = $32 thousand per month.
(c) The policy removes a DWL of $50 thousand. Net gain = 50 − 35 = $15 thousand per month, so it is worth doing. If running the regulator cost $60 thousand, the net gain would be 50 − 60 = −$10 thousand: the cure would cost more than the problem, so society would be better off without it.
Check. Total surplus at 40 is $800 thousand. At 30 it is 800 − 50 = $750 thousand; at 48 it is 800 − 32 = $768 thousand. Both are smaller, as they must be.
Common misconceptions
- “Efficient means fair.” Efficiency is about the total size of surplus, not its distribution.
- “The efficient quantity is the largest quantity possible.” Producing more than Q* creates a DWL, just as producing less does.
- “Every market equilibrium is efficient.” As a rule, only a competitive market in which buyers and sellers bear all the benefits and costs reaches Q*. (A monopoly that can perfectly price discriminate, Topic 4.3, is a special exception.)
- “A monopoly underproduces because it is greedy or careless.” It is acting rationally on private incentives. The problem is the incentive, not the motive.
- “DWL is the monopoly’s extra profit.” Profit taken from buyers is a transfer. DWL is surplus nobody gets.
- “DWL is the gap between MSB and MSC at one quantity.” That gap is the loss on the last unit only. DWL adds up the losses on all the units between Q and Q*, so it is an area (½ × base × height for straight lines).
- “Any policy that reduces DWL is a good policy.” Its benefit must be weighed against its cost.
Where this leads
Next, Topic 6.2 looks closely at one cause of inefficiency, external costs and benefits: Externalities study guide. To review the previous topic, see the Topic 5.4 study guide. Test yourself with the practice questions, then use the revision notes and the checklist to consolidate.
Quick questions
Is a socially efficient outcome also a fair outcome?
Not necessarily. Efficiency is about making total surplus as large as possible. It says nothing about who receives that surplus. How income is shared is a separate question (Topic 6.5).
Why does producing too much create a loss? Surely more output is good.
Each unit beyond the efficient quantity costs society more to make than it is worth to anyone. Making it uses resources that would create more value elsewhere, so total surplus falls.
Is deadweight loss the extra profit a monopoly earns?
No. Extra profit taken from buyers is a transfer: one group loses and another gains. Deadweight loss is surplus that nobody receives, because units worth more than they cost are never made.