Market Failure: Externalities and Public Goods
Some costs and benefits land on people outside the deal, so a market makes too much of what pollutes and too little of what spills over as a benefit. Public goods sit further out still: nobody can be shut out and one person's use does not use them up, so no seller can charge for them and taxes fund them instead.
What a learner can do afterwards
- Compare private cost with social cost for a polluting factory and show why the market quantity is too high
- Choose a tool for a stated failure (a tax, a subsidy, a tradable permit, a rule) and justify the choice
- Explain free riding and why street lighting is not sold lamp by lamp
1 · Read
A factory that fouls a river pays for labour and machines but not for the dead fish downstream. Its private cost sits below the true social cost, so it produces more than society wants. This gap is called a negative externality, and the market quantity ends up too high. The fix pushes private cost toward social cost: a tax prices the harm, a tradable permit caps it, a rule bans the worst of it.
Some goods refuse to be sold lamp by lamp. Street lighting is nonrival, since your use leaves just as much light for me, and nonexcludable, since nobody can fence the glow off from passers-by. Private sellers cannot charge everyone who benefits, so they supply too little or nothing. That is why councils fund street lights, lighthouses, and flood defences from taxes instead.
A free rider enjoys the benefit while skipping the bill. One family refuses to chip in for shared security patrols, knowing the patrols pass their street anyway. If everyone reasons this way, the patrols never get funded even though all want them. Small groups can shame riders into paying, but big groups usually need taxes or rules.
Benefits spill over too. A beekeeper's hives pollinate the orchard next door for free, and vaccines shield even the unvaccinated. Each case tempts free riding, so markets supply too little alone. Subsidies, public funding, or direct provision close the gap. Spot the spillover and you have found the policy target.
Price the harm, fund what cannot be fenced, and expect riders wherever payment is voluntary.
2 · Watch
Take it off screen
Where it sits
Where this leads
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.