---
title: "Externalities and Spillovers"
description: "Choices spill over onto people who had no say: a factory's smoke, a neighbour's loud music, a planted garden, a vaccination; why harmful spillovers get overproduced and helpful ones underproduced when"
canonical: https://lightmysky.com/learn/civics-and-economics/externalities-and-spillovers-mt_F4SQsGpZk9
source: https://lightmysky.com/learn/civics-and-economics/externalities-and-spillovers-mt_F4SQsGpZk9.md
retrieved: 2026-10-19
---

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# Externalities and Spillovers

Choices spill over onto people who had no say: a factory's smoke, a neighbour's loud music, a planted garden, a vaccination; why harmful spillovers get overproduced and helpful ones underproduced when nobody pays or is paid for them

Subject: Civics & Economics · Area: Economics · Ages 10 to 13
Page: https://lightmysky.com/learn/civics-and-economics/externalities-and-spillovers-mt_F4SQsGpZk9

## Ready when they can

- Identify the third party affected by a choice (e.g., 'the whole street breathes the barbecue smoke')
- Sort spillovers into helpful and harmful (e.g., a flower garden on the street vs. litter left at the park)
- Suggest a way to handle a spillover (e.g., a rule, a fine or a reward, like a tax on pollution or a subsidy for bee-keeping)

## Lesson: When Your Choices Touch Other People

Some choices only affect you. Others spill over onto people who had no say at all. Economists call this a spillover, or an externality. It happens any time your choice makes someone else's life better or worse, without them choosing it.

**Example.** Think about a backyard barbecue. The person grilling chose to light the fire. But the whole street breathes the smoke, whether they wanted to or not. The neighbors never agreed to smell smoke tonight. That is a spillover.

*(drawing: The factory only pays a little, but the town downstream pays the most. That is why the factory keeps polluting: it isn't the one bearing the biggest cost.)*

**Tip.** Not every spillover is bad. A flower garden on your street lifts everyone's mood for free. That is a helpful spillover. Litter left at the park makes the park worse for every visitor. That is a harmful one. Sort each spillover by asking: does it help or hurt the people who never chose it?

When nobody pays for a harmful spillover, it keeps happening, because the person causing it never feels the cost. When nobody gets paid for a helpful spillover, it happens less than it could, because the person creating it never gets credit. A fine on pollution or a reward for bee-keeping can fix that by connecting the cost or benefit back to the person who caused it.

**Recap.** A spillover touches people who never chose it: harmful ones need a cost attached, helpful ones need a reward.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Incentives and Choices](https://lightmysky.com/learn/civics-and-economics/incentives-and-choices-mt_KxkoDkbjPy)
- [How the Economy Works](https://lightmysky.com/learn/life-skills/how-the-economy-works-mt_udgPy5oAvR)

## Opens up

- [Market Failure: Externalities and Public Goods](https://lightmysky.com/learn/civics-and-economics/market-failure-externalities-and-public-goods-mt_TvBDQJqv1R)