Economic decisions should be forward-looking. In microeconomics, ignoring sunk costs helps consumers and firms choose the option that maximizes net benefits using only costs and benefits that can still change.

Core Idea: Past Costs Don’t Change the Best Choice

A rational decision-maker compares future (avoidable) costs and future benefits across options. If a cost has already been incurred and cannot be recovered, it should not affect what you do next, because it is the same no matter which option you choose.

Sunk Costs vs. Relevant Costs

A sunk cost is irrelevant to the choice at hand because it does not differ across the available actions going forward.

Conceptual illustration accompanying the definition of a sunk cost: a cost already incurred that cannot be recovered. This visual reinforces the decision-making principle that only future, changeable (avoidable) costs and benefits should enter the comparison across options. It pairs well with the definition-and-application structure of the notes. Source

Relevant costs are avoidable (they can be reduced by choosing a different action), and they are the ones that matter for optimal choice.

The Sunk Cost Fallacy

The sunk cost fallacy is the tendency to “throw good money after bad” by continuing an activity just because you have already invested resources in it. In AP Micro, this is treated as irrational because the decision should depend only on what changes from this point onward.

Decision Rule for Ignoring Sunk Costs

When choosing whether to continue, expand, reduce, or stop an activity, evaluate only incremental (additional) consequences from now on:

• Identify the alternatives you can still choose.

• List future benefits of each alternative.

• List future costs that will occur only if you choose that alternative.

• Ignore any past expenditures (money, time, effort) that cannot be recovered.

• Choose the alternative with the greatest net benefit (or, equivalently, the lowest net cost for a given benefit).

Net benefits formalize the forward-looking comparison:

In applying this, sunk costs belong to “total cost already paid,” but they do not belong in the comparison of options because they do not vary with the current decision.

How This Shows Up for Firms and Consumers

Firms

For a firm, sunk costs commonly include:

• Past research and development spending

• Money already spent on specialized equipment that has no resale value

• Nonrefundable fees already paid

A firm should continue producing or operating only if future revenues justify future costs.

Past fixed commitments that cannot be changed should not keep an unprofitable operation running.

Consumers

For consumers, sunk costs commonly include:

• Nonrefundable tickets or memberships already purchased

• Time already spent waiting in line

• Past spending on a hobby or course that no longer yields benefits

A consumer should allocate remaining time and money to the option that provides the highest additional benefit relative to additional cost, not to “get their money’s worth” from an unrecoverable purchase.

Common Pitfalls AP Students Should Avoid

• Treating sunk costs as if they are “recovered” by continuing; continuing cannot change the past payment.

• Confusing sunk costs with fixed costs: some fixed costs can still be avoidable in the long run (e.g., a lease that ends), so what matters is whether the cost can be changed from the decision point.

• Mixing accounting intuition (“we must cover what we spent”) with economic choice: optimal decisions focus on what changes across options, not on what has already happened.

• Using fairness or emotion as a criterion; microeconomic rationality assumes choices aim to maximize net benefits given constraints, regardless of past commitments.