Negative Consumption Externalities

Why Private Consumption Can Be Excessive

A person choosing whether to make one more trip, use a noisy device, or smoke indoors typically considers the benefit to themself and the costs they personally face. But the choice can also impose a cost on people who are not part of the transaction. Traffic delays imposed on other drivers and smoke inhaled by nearby people are examples of negative consumption externalities.

When consumers do not account for these effects, the market quantity reflects private rather than social benefits. The market can therefore provide more of the activity than is efficient. To see why, distinguish the benefit to the consumer from the total benefit to everyone affected, and compare each with the relevant marginal cost.

The basic comparison in this tutorial assumes that production itself creates no externality. Thus, marginal private cost (MPC), the additional cost borne by the producer, is also marginal social cost (MSC). If production causes a separate external harm or benefit, the cost side must also be adjusted; that is not the case considered here.

In a standard competitive market with no consumption externality recognized by buyers, the market quantity \(Q_m\) is where \(MPB=MPC\). The efficient quantity \(Q_e\), which accounts for the effects on third parties, is where \(MSB=MPC\). If MEC is positive at the relevant quantities and the curves have the usual slopes, MSB lies below MPB, and the efficient quantity is below the market quantity.

Reading the Demand-and-Cost Diagram

Put quantity on the horizontal axis and marginal dollars per unit on the vertical axis. The downward-sloping demand curve represents MPB. The upward-sloping cost curve represents MPC and, under the assumption above, MSC. Since \(MSB=MPB-MEC\), the social-benefit curve lies below the demand curve wherever MEC is positive. The market quantity is the intersection of MPB and MPC; the efficient quantity is the intersection of MSB and MPC.

At the market quantity, consumers’ private marginal benefit equals marginal cost. But if there is a negative consumption externality, the benefit to society is lower than the private benefit by the amount of MEC. Consequently, at \(Q_m\), \(MSB<MPC\): the last units consumed cost society more than the benefit they provide. Reducing consumption from the market quantity can improve total surplus until the efficient quantity is reached.

Why the Efficient Quantity Satisfies the Social Condition

The intersection \(MSB=MPC\) follows from how an additional unit changes total surplus. Assume quantities are continuous, marginal curves are well-defined, and each unit’s private benefit, production cost, and external consumption harm can be added across units. For a quantity \(Q\), write total surplus, apart from any fixed terms, as the sum of private benefits minus production costs and external harms:

Differentiating with respect to quantity gives the change in surplus from one more unit:

At an interior maximum of total surplus, a small increase or decrease in quantity must not improve surplus. Therefore \(dW/dQ=0\), which implies \(MSB(Q_e)=MPC(Q_e)\). If \(MSB>MPC\), the next unit adds more social benefit than cost, so increasing quantity raises surplus. If \(MSB<MPC\), the next unit’s social cost exceeds its benefit, so decreasing quantity raises surplus. With the usual downward-sloping MSB and upward-sloping MPC, the intersection identifies the unique interior maximum. If quantities are constrained or the curves do not cross within the feasible range, the efficient choice may instead be at a boundary.

The market condition is different: buyers and sellers respond to private benefits and costs, so the competitive market quantity satisfies \(MPB=MPC\). Unless the external cost is zero, that condition does not in general maximize total surplus.

Worked Examples in Other Settings

Common Mistakes and Practical Interpretation

The key comparison is between marginal values at the same quantity. The vertical distance from MPB to MSB at \(Q\) is MEC at \(Q\). It is not the total harm accumulated across all units. If MEC changes with quantity, the distance between the curves changes too.

A frequent error is to find the market intersection \(MPB=MPC\) and call it efficient. That is correct only when the relevant marginal external cost is zero (or when other conditions exactly offset it). For example, in the secondhand-smoke calculation, the market quantity is 15 because \(MPB=MPC=27\). But at that same quantity, \(MEC=19\), so \(MSB=8\), not 27. Since \(8<27\), the last market unit is socially costly on net.

Another error is to subtract total external harm from a marginal benefit. In the smoke example, total external harm through \(Q\), under the stated marginal-cost curve, would be the integral of \(4+x\) from 0 to \(Q\). At \(Q=15\), this is \(4(15)+15^2/2=172.5\) dollars. That total is not the marginal external cost at \(Q=15\), which is \(4+15=19\) dollars per session. Marginal benefit and marginal cost curves must be compared in the same per-unit terms.

The graphical conclusion is not that every unit of the activity is undesirable. Units before the efficient quantity can provide social benefits greater than their costs. The problem is the additional consumption between \(Q_e\) and \(Q_m\), where social marginal benefit falls below marginal cost. This distinction is useful whenever private consumption affects others, including congestion, secondhand smoke, and disruptive noise.