Individual skill
s4h-economics-externalities
Identifies costs and benefits not borne by the decision-maker — and how to internalise them. Triggers: 'externality', 'who else is affected', 'costs falling on others', 'side effects', 'spillover', 'the market isn't accounting for this', 'negative externality', 'positive externality', 'social cost vs. private cost', 'Pigou', any situation where the person deciding does not bear all the consequences.
skills/s4h-economics-externalities/SKILL.md
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# Economics: Externalities
A.C. Pigou's insight, formalised in *The Economics of Welfare* (1920): when the person making a decision does not bear all of its costs — or capture all of its benefits — they will systematically make the wrong decision. The factory owner who pollutes a river imposes costs on everyone downstream but pays none of them. The individual who gets vaccinated reduces the risk of infection for everyone around them but captures only a fraction of the total benefit. In both cases, the divergence between private cost-benefit calculation and social cost-benefit calculation produces a predictable market failure.
This is not a moral failure — it is a structural one. The decision-maker is not evil for failing to account for costs they do not bear; they are responding rationally to their situation. Pigouvian analysis makes the structural problem visible: the social optimum and the private optimum diverge whenever externalities exist. And wherever they diverge, there is scope for a policy or institutional intervention that realigns them — tax (for negative externalities), subsidy (for positive externalities), property rights (Coase's alternative), or social norms and regulation.
Ronald Coase's counter-argument (the Coase Theorem) adds a crucial qualification: when transaction costs are low and property rights are clear, private negotiation between parties can internalise externalities without government intervention — and produce the efficient outcome regardless of who holds the initial property right. In practice, transaction costs are often high, so Coase is a theoretical benchmark rather than a practical solution. But identifying whether transaction costs are low is a crucial diagnostic step.
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## Your Process
**Step 1: Identify the decision and the decision-maker**
Clarify what decision is being made, who is making it, and what the decision-maker's cost-benefit calculation looks like from their perspective alone. Establish the baseline: what does the decision-maker see as costs and benefits?