Sunk Cost Fallacy vs Prospect Theory
Sunk Cost Fallacy and Prospect Theory are two Behavioral Economics concepts in AP Economics that students often mix up. The sunk cost fallacy is continuing an endeavor because of money or effort already spent, even when it is no longer worthwhile. Prospect theory describes how people choose among risky options based on perceived gains and losses relative to a reference point, not final wealth. Here is how they compare side by side.
Rational decisions should ignore sunk costs (which can't be recovered) and weigh only future costs and benefits. People fall into this trap because of loss aversion and a reluctance to 'waste' past investment.
Developed by Kahneman and Tversky, it shows people weight losses more than gains (loss aversion) and overweight small probabilities. It explains many real choices that expected-utility theory cannot.
Sunk Cost Fallacy vs Prospect Theory: A Named Error and the Model That Predicts It
| Sunk Cost Fallacy | Prospect Theory | |
|---|---|---|
| What kind of claim it makes | Normative, it says this reasoning is a mistake | Descriptive, it says this is how choices actually run |
| What sets it off | An unrecoverable outlay sitting in the past | Any risky choice read against a reference point |
| Does risk have to be present? | No, a certain continuation cost is enough | Yes, the model is built out of gambles and probabilities |
| Role of the reference point | No part of the idea itself, though it explains the pull | The whole spine of the model, every outcome is a gain or a loss from it |
| What it says about doubling down | Counting money already gone is an error | People turn risk seeking once they are behind |
| The correction it offers | Ask whether you would start this today | None, a description of behavior gives no advice |
| Where an exam tests it | Marginal analysis, continue or shut down questions | Choice under risk, framing and reference dependence |
One of these can be wrong on an exam, the other can only be a bad description
A student can commit the sunk cost fallacy and lose the point, because the fallacy is a rule about which figures belong in a comparison and unrecoverable spending never belongs. Prospect theory cannot be committed at all. A model of behavior is either a good fit for what people do or a poor one, and no answer key marks a chooser wrong for having a reference point. Keeping that difference straight also keeps the two from collapsing into each other, because each one runs in cases where the other has nothing to say. Certainty strips prospect theory out: a shop that has already paid 90 dollars in permit fees and now faces a guaranteed 40 dollar renewal against a guaranteed 55 dollars of extra trade has no gamble anywhere in it, only a marginal comparison that says renew, and letting the 90 dollars in either direction is the fallacy. A blank slate strips the fallacy out: someone choosing between a certain 30 dollars and a coin flip paying 70 or nothing has spent nothing and cannot be dragged by a past outlay, yet their answer usually flips depending on whether the same money is described as winnings or as a fine already levied. See /glossary/marginal-analysis for the comparison the fallacy corrupts.
Being behind converts a careful manager into a gambler, and the felt numbers show the swap
Prospect theory earns its keep here by explaining a pattern the fallacy only labels. A workshop has sunk 320 dollars into a prototype. Cancelling books a certain loss of 320. Launching costs 150 more and returns 300 dollars with probability four in ten and nothing otherwise. Marginal analysis settles it in one line: expected extra revenue is four tenths of 300, or 120 dollars, against 150 dollars of extra cost, so launching burns another 30 dollars and the workshop should cancel. Now price the same two branches the way prospect theory does, in losses from a reference point of breaking even. Cancelling is a sure loss of 320. Launching is a loss of 170 with probability four tenths and a loss of 470 with probability six tenths, which averages to 350, exactly the 30 dollars worse the marginal test found. Give the value function diminishing sensitivity so that felt pain runs 100 units at a 170 dollar loss, 150 units at 320 and only 180 units at 470. The gamble now feels like four tenths of 100 plus six tenths of 180, which is 148 units, just under the 150 units of certain pain. The manager takes the worse bet because a bigger loss barely hurts more, and that convexity, not stubbornness, is the mechanism.
Frequently asked questions
Is the sunk cost fallacy part of prospect theory?
Prospect theory supplies a mechanism for the fallacy rather than containing it as a definition. The fallacy is the normative claim that unrecoverable spending must be left out of a decision. Prospect theory is a descriptive model in which outcomes are valued as gains and losses from a reference point, with sensitivity to size falling off as losses grow. That falling sensitivity makes a risky continuation feel cheaper than a certain write off, which is why the fallacy is so common. Each idea survives without the other.
Why does prospect theory predict throwing good money after bad?
Once a project falls behind, every branch of the decision sits in the loss region of the value function, where the curve is convex and each extra dollar of loss stings less than the one before it. Cancelling locks in a certain loss at full felt weight. Continuing offers a chance, however slim, of a smaller final loss, and the downside costs little extra in felt terms. Risk seeking in losses follows directly, which is the mirror image of the caution the same person shows over gains.
Which of the two shows up on an AP Economics exam?
Sunk costs carry the exam weight. Continue or shut down questions, profit maximization and marginal analysis all depend on excluding spending that cannot be recovered, and the trap is usually a stem that quotes a large past outlay to see whether you use it. Prospect theory sits outside the AP course description and belongs to behavioral and college level treatments of risk, so it explains why students fall for the trap rather than appearing as a graded concept itself.
Related comparisons
Get AP Econ exam tips in your inbox
Occasional emails with study tips, new interactive graphs, and exam-season reminders. Free, no spam.
No spam. Unsubscribe anytime. Read our privacy policy.
Keep track of what you have studied
A free EconLearn account adds progress tracking, your quiz history, and achievements. Studying here is free either way, and there is nothing to pay for as a student.
Create a free accountAlready have one? Sign in
Last updated