Externalities

• Externalities are effects of an economic transaction that land on a third party — someone who wasn't the buyer or the seller.
• They can be negative (a cost dumped on someone else) or positive (a benefit given to someone else "for free").
• They can happen on the production side (how something is made) or the consumption side (how something is used).
• Private cost/benefit = what the person directly involved pays or gains. External cost/benefit = the bit that spills onto everyone else. Add them together and you get the social cost or benefit — the true cost/benefit to society as a whole.
• Negative externalities of production → the market over-produces (too much of the harmful thing gets made, too cheaply).
• Positive externalities of consumption → the market under-consumes (too little of the good thing gets used, because people only weigh up their own benefit).
• Governments step in with things like taxes, regulation, subsidies, and building more provision to try to correct these market failures — but every intervention has trade-offs.

Think about it like this: normally an economic transaction only involves two people — a buyer and a seller. You buy a coffee, the café sells you a coffee. Simple. Just the two of you are affected.

But sometimes a transaction reaches further than that and touches a third party — someone who had absolutely nothing to do with the deal, didn't agree to it, and often doesn't even know it's happening. That knock-on effect on the third party is the externality, also called a spillover effect (because the effect "spills over" the edge of the original deal, like water overflowing a glass).

Externalities come in two flavours, and along two different "sides" of the market:

This is the part students most often mix up, so let's slow right down.

Private cost is simply what the producer actually pays out of their own pocket to make something — wages, raw materials, electricity bills. It's the cost they feel.

External cost is the damage that gets caused but is not paid for by the producer — it's dumped on someone else instead. A steel factory doesn't pay for the extra asthma inhalers that nearby residents now need because of the air pollution it created. That cost still exists — it's just been pushed onto other people, almost like the factory quietly making someone else pick up part of its bill.

Social cost true, full

The exact same logic flips over for benefits:

Private benefit

External benefit