Part (a): consumer surplus, producer surplus and allocative efficiency [10]
Defining the terms
Consumer surplus is the difference between the maximum price a consumer is willing and able to pay for a good, read off the demand curve, which is the consumer's marginal benefit schedule, and the price they actually pay. Producer surplus is the difference between the price a producer receives and the minimum price they would have been willing to accept, read off the supply curve, which is the producer's marginal cost schedule. The sum of the two is community surplus, the total net welfare the market generates. Allocative efficiency is the situation in which society's scarce resources are used to produce the combination of goods that maximises welfare, which occurs where marginal benefit equals marginal cost.
How the surpluses arise at equilibrium
For every unit up to the equilibrium quantity, the value consumers place on the unit, shown by the demand or marginal benefit curve, exceeds the market price, so buyers gain. Summed over all those units, consumer surplus is the area above the equilibrium price and below the demand curve. For every unit up to the equilibrium quantity, the market price exceeds the minimum the producer would accept, shown by the supply or marginal cost curve, so sellers gain. Summed, producer surplus is the area below the equilibrium price and above the supply curve. Community surplus is the sum of the two, the whole area between the demand and supply curves up to the equilibrium quantity.
Why the equilibrium is allocatively efficient
The link to the best allocation is the condition that marginal benefit equals marginal cost. At the competitive equilibrium, demand (marginal benefit) equals supply (marginal cost). For every unit up to the equilibrium quantity, marginal benefit exceeds marginal cost, so producing it adds more to society's benefit than to its cost, and it raises total surplus. Beyond the equilibrium quantity, marginal cost would exceed marginal benefit, so producing more would cost society more than the benefit it yields, reducing welfare. The equilibrium quantity, where marginal benefit equals marginal cost, is therefore the one at which community surplus is maximised: no reallocation could raise total welfare. This is the definition of allocative efficiency, the right goods produced in the right amounts.
Describing the diagram in prose
Picture price on the vertical axis and quantity on the horizontal axis, with a downward-sloping demand curve (marginal benefit) and an upward-sloping supply curve (marginal cost) crossing at the equilibrium price and quantity. Consumer surplus is the triangle above the equilibrium price and below the demand curve; producer surplus is the triangle below the equilibrium price and above the supply curve. Their sum, community surplus, is the whole area between the two curves up to the equilibrium quantity, and it is at its largest exactly at the equilibrium, because moving away from it in either direction shrinks the combined area.
Link back
At the free-market equilibrium, where marginal benefit equals marginal cost, consumer and producer surplus together are maximised, so community surplus is maximised. That is precisely the condition for allocative efficiency, which is why the competitive equilibrium is described as the best allocation of resources.
Part (b): does the free market always allocate resources efficiently? [15]
Introduction
The part (a) result, that the competitive equilibrium maximises community surplus and achieves allocative efficiency, holds only under strict assumptions: that there are no externalities, no public goods, no market power and no information failures, and that society does not object to the resulting distribution. A market failure is any situation in which the free market produces an allocatively inefficient outcome. This answer discusses whether the free market always allocates resources efficiently, and argues that under the textbook conditions it does, but that those conditions frequently fail, so in the real world the market often misallocates resources and the answer is no.
Externalities
When production or consumption imposes costs or confers benefits on third parties, private valuations diverge from social ones, and the market quantity differs from the social optimum. A coal-fired power station that pollutes faces a marginal social cost above its marginal private cost, so the market over-produces electricity and leaves a welfare loss; the global market for carbon-intensive energy is the clearest case, where unpriced emissions drive output beyond the level that maximises social welfare. The surplus calculation of part (a) silently assumed marginal private benefit equalled marginal social benefit and marginal private cost equalled marginal social cost; once they diverge, the equilibrium no longer maximises social surplus.
Evaluation. The size of the misallocation depends on the size of the externality. For a good with a trivial spillover the market is close enough to efficient, but for carbon emissions, where the external cost is large and global, the misallocation is severe, which is why so many governments have introduced carbon pricing.
Public goods and market power
Public goods such as national defence or a lighthouse are non-rivalrous and non-excludable, so the free-rider problem means the market supplies too little or none at all, a complete failure of allocation rather than a marginal distortion. Market power is the other major case: a monopolist or a cartel restricts output and raises price above marginal cost, so marginal benefit exceeds marginal cost at the quantity sold and a deadweight welfare loss results. The OPEC oil cartel illustrates the point, restricting output to hold prices above the competitive level, which transfers surplus from consumers to producers and shrinks total surplus relative to the competitive outcome.
Evaluation. These failures are not symmetric in severity. A natural monopoly may still be the cheapest way to supply a good because of economies of scale, so the efficient response is regulation rather than forced competition, and some apparent market power is competed away over time as new entrants or substitutes appear.
Asymmetric information and equity
Where one side of a transaction knows more than the other, the market can allocate badly: in the second-hand car market, buyers cannot tell good cars from bad, so they offer a price that drives the best cars out and leaves a market dominated by poor ones, the classic problem of adverse selection. Finally, even a perfectly efficient market says nothing about fairness. The equilibrium maximises total surplus given the existing distribution of income, but that distribution may be highly unequal, so the market may allocate resources efficiently towards the wants of the rich while the basic needs of the poor go unmet. Efficiency and equity are different criteria, and the free market guarantees only the first.
Evaluation. Information failures can sometimes be solved privately, through warranties, branding or third-party certification, without government action, so they do not always justify intervention. The equity objection is a value judgement rather than an efficiency claim, but it is a powerful reason why societies rarely leave allocation entirely to the market.
Judgement
Under the strict conditions of part (a), no externalities, no public goods, no market power, perfect information and an accepted distribution, the free market does allocate resources efficiently, and that result is the benchmark against which everything else is judged. But those conditions are demanding, and real markets routinely violate them. Externalities pervade energy and the environment, public goods will not be supplied at all, market power restricts output, information is unequal, and the market is blind to equity. The supported judgement is therefore that the free market allocates resources efficiently only in the special case where its assumptions hold, and that in practice it frequently misallocates, which is the standing justification for government intervention, though that intervention must itself be judged against the risk of government failure rather than assumed to be costless.