Positive externalities create underconsumption or underproduction

Social marginal benefit exceeds private marginal benefit.

Education, vaccination, and research may benefit third parties. If private demand reflects only the consumer’s benefit, MSB lies above demand and the market quantity is too low. A corrective subsidy equal to marginal external benefit at the efficient quantity can align private and social incentives.

Choose the missing marginal value

A factory’s private marginal cost at the efficient quantity is $18 and each unit causes $5 of marginal external damage. MSC is $23. A $5 per-unit tax makes the factory face the social marginal cost at that quantity.

A positive externality exists when an action creates a benefit for a third party and the decision maker cannot capture that benefit in the market price. A vaccinated person receives private protection, while people nearby may face lower infection risk. A firm that develops a technique may teach employees or reveal knowledge that other firms later use. Because the buyer or producer is not paid for the entire social gain, the private market provides too little.

For a positive consumption externality, ordinary demand records marginal private benefit. Add the marginal external benefit to obtain social marginal benefit:

MSB=MPB+MEB.

MSB lies above private demand, and the efficient quantity where MSB meets marginal social cost exceeds the market quantity. For a positive production spillover, social marginal cost may instead lie below private marginal cost. The same underproduction conclusion can arise through a different curve, so the source of the spillover still matters.

Calculate the subsidy benchmark

A training course gives a student $700 of marginal private benefit and nearby employers $200 of marginal benefit through a more skilled labor pool. At that enrollment level, MSB=$900. If marginal cost is $900, the unit is socially efficient even though the student would not buy it at a $900 price. A $200 subsidy can bridge the private-social gap at that quantity.

A subsidy is a means, not a free gain. Raising public revenue can create administrative and incentive costs, and a poorly targeted subsidy can pay for activity that would have occurred anyway. Patents take another approach to knowledge spillovers by granting temporary exclusion so innovators can capture more benefit. That can encourage research but also creates temporary market power. Public provision, prizes, and grants represent other tradeoffs.

Do not say that a positive externality shifts market demand by itself. The observed private demand curve is precisely what omits the external benefit. Analysts construct an additional social-benefit curve to evaluate efficiency. Also distinguish a third-party spillover from ordinary consumer surplus: the buyer’s own benefit above price is internal to the transaction, not an externality.

The reliable sequence is: identify the outsider, determine whether the effect is a benefit or cost, place it on the correct side of the market, compare Q_m with Q^*, and only then choose a policy. That reasoning is more dependable than memorizing “positive means subsidy.”