What Is the Sunk Cost Fallacy?

What Is the Sunk Cost Fallacy?

The sunk cost fallacy is the tendency to continue investing in a losing course of action because of previously invested resources. You start reading a book that turns out to be bad, but you keep reading because you have already invested two hours. You stay in a failing relationship because you have already invested years. You continue funding a project that is clearly failing because you have already spent millions. In each case, the past investment — the sunk cost — is driving the decision, even though it should not.

The rational principle is straightforward: past investments are sunk — they cannot be recovered. Whether you have invested two hours or two years, the question for the future is the same: do the expected future benefits justify the expected future costs? The past is irrelevant to this calculation, because the past cannot be changed. If the future costs outweigh the future benefits, you should stop — regardless of how much you have already invested.

This principle is clear in theory but difficult in practice. People consistently let sunk costs influence their decisions, and this tendency — the sunk cost fallacy — is one of the most robust and well-documented biases in human decision-making. It affects individuals, organizations, and governments, leading to wasted resources, missed opportunities, and escalating commitment to failing projects.

Historical Background

Economic Theory

The concept of sunk costs is central to economic theory. In microeconomics, the principle is that rational agents make decisions at the margin: they compare the marginal (additional) cost of an action with its marginal (additional) benefit. Past costs are irrelevant to this comparison because they are sunk — they cannot be recovered regardless of what the agent does now.

This principle is taught in every introductory economics course, but it is routinely violated in practice. The gap between the economic ideal and actual behavior was documented by behavioral economists in the late twentieth century, who showed that people systematically deviate from rational-choice predictions in ways that are predictable and consistent.

The Concorde Fallacy

The sunk cost fallacy is sometimes called the Concorde fallacy, after the supersonic jet project jointly funded by the British and French governments. By the early 1970s, it was clear that the Concorde would not be commercially viable — the development costs were far higher than projected, and the market for supersonic travel was smaller than expected. But the governments continued funding the project, citing the enormous investments already made. The project went forward, losing money for decades, because the governments could not bring themselves to abandon something they had invested so much in.

The Concorde is a dramatic example, but the pattern is common. Businesses continue funding failing products because of development costs already sunk. Governments continue wars because of the lives and money already lost. Individuals continue bad investments because of the money already committed. In each case, the past investment is driving the decision, not the future prospects.

Behavioral Economics

The sunk cost fallacy was studied systematically by behavioral economists and psychologists beginning in the 1980s. Hal Arkes and Catherine Blumer, in a 1985 paper, documented the fallacy in a series of experiments. In one, participants who had paid for a ski trip were more likely to attend it when it conflicted with a more enjoyable alternative, because they had already paid. The sunk cost of the payment influenced their decision, even though the money was lost either way.

Richard Thaler and other behavioral economists incorporated the sunk cost fallacy into the broader framework of behavioral economics — the study of how psychological factors cause systematic deviations from rational-choice predictions. Thaler's concept of mental accounting — the tendency to categorize money by its source or intended use, rather than treating all money as fungible — helps explain why sunk costs influence decisions: people treat money already invested as belonging to a particular "account" and are reluctant to close that account at a loss.

Key Thinkers

Hal Arkes and Catherine Blumer

Arkes and Blumer's 1985 paper, "The Psychology of Sunk Cost," provided the first systematic experimental evidence of the sunk cost fallacy. They demonstrated the effect across multiple domains — including business decisions, personal investments, and entertainment choices — and showed that the fallacy is robust and general.

Arkes and Blumer also identified a key mechanism: the desire to avoid waste. People continue investing in failing projects because abandoning them feels like wasting the prior investment. This feeling of waste is psychologically painful — more painful, often, than the continued loss from continuing the project. The fallacy is driven not by a calculation error but by an emotional response: the avoidance of the feeling of having wasted resources.

Daniel Kahneman and Amos Tversky

Kahneman and Tversky's prospect theory (1979) provides the theoretical framework for understanding the sunk cost fallacy. Prospect theory describes how people make decisions under uncertainty, and one of its key findings is loss aversion: people feel losses more intensely than equivalent gains. Losing $100 feels worse than gaining $100 feels good.

Loss aversion explains the sunk cost fallacy because abandoning a project in which one has invested converts the investment from a "paper loss" (which can still be rationalized as potentially recoverable) into a "realized loss" (which must be acknowledged as final). The psychological pain of realizing the loss is greater than the pain of continuing to lose slowly, so people continue rather than abandon. The fallacy is a form of loss aversion: the pain of acknowledging the sunk cost as lost drives the decision to continue.

Richard Thaler

Thaler's work on mental accounting and behavioral economics provided the economic framework for the sunk cost fallacy. Thaler argued that people do not treat money as fungible (as rational-choice theory assumes) but categorize it into mental accounts based on its source, intended use, or prior commitment. Money already invested in a project is assigned to that project's account, and closing the account at a loss feels like a failure, even though the money is lost regardless.

Thaler's work has been influential in both economics and public policy, showing that understanding behavioral biases like the sunk cost fallacy is essential for designing effective policies and institutions.

Mechanisms Behind the Fallacy

Loss Aversion

The primary mechanism is loss aversion: the psychological tendency to feel losses more intensely than equivalent gains. Abandoning a sunk cost means acknowledging a loss, which is psychologically painful. Continuing the investment avoids the immediate pain of acknowledgment, even if it leads to greater losses in the long run.

Waste Aversion

People have a strong aversion to waste. Abandoning a project in which resources have been invested feels like wasting those resources — even though the resources are already spent and cannot be recovered. The feeling of waste is so aversive that people will continue investing to avoid it, even when the investment is itself wasteful.

Desire to Appear Consistent

People want to appear consistent — to themselves and to others. Abandoning a project in which one has invested is an admission that the initial investment was a mistake. This admission is embarrassing, especially if the investment was publicly defended. Continuing the project avoids the embarrassment of admitting error.

Identity and Self-Image

When a person's identity is tied to a project, abandoning it is not just a financial decision but an identity crisis. A founder who abandons their startup is abandoning their identity as a founder. A researcher who abandons a line of inquiry is abandoning their identity as a researcher of that topic. The sunk cost fallacy is reinforced by the threat to identity that abandonment represents.

Escalation of Commitment

The sunk cost fallacy often leads to escalation of commitment: the tendency to invest increasing amounts in a failing course of action. Each additional investment creates a new sunk cost, which makes abandonment even more painful, which leads to further investment. The cycle can continue until the resources are exhausted.

The Sunk Cost Fallacy in Practice

Business Decisions

The sunk cost fallacy is endemic in business. Companies continue funding failing products, persist with unsuccessful strategies, and retain underperforming employees because of the investments already made. The fallacy is particularly dangerous in large organizations, where the decision to abandon a project involves admitting error to superiors, colleagues, and shareholders — a cost that decision-makers are reluctant to pay.

Public Policy

Governments are particularly vulnerable to the sunk cost fallacy because the costs of abandonment are political as well as financial. A government that abandons a public project must explain to taxpayers why their money was wasted — a political cost that may exceed the cost of continuing the project, even when continuing is objectively worse. The Concorde is the paradigmatic example, but the pattern recurs in infrastructure projects, military engagements, and social programs.

Personal Relationships

The sunk cost fallacy affects personal relationships: people stay in unhappy relationships because of the time and emotional energy already invested. The question "should I continue?" is answered by reference to the past ("we have been together for five years") rather than the future ("will the next five years be better or worse?"). The fallacy leads to prolonged unhappiness as people refuse to "waste" their prior investment.

Education and Career

Students continue in programs they dislike because they have already invested years. Workers stay in careers that make them unhappy because they have already invested in training and experience. The sunk cost fallacy leads people to make decisions based on what they have already done rather than on what would make them happy or fulfilled in the future.

Overcoming the Sunk Cost Fallacy

Ask "Would I Start This Today?"

The most effective technique for overcoming the sunk cost fallacy is to ask: "If I were making this decision for the first time today, with no prior investment, would I choose this option?" If the answer is no, the prior investment is driving the decision, and the rational choice is to stop.

Separate the Decision from the Investment

Explicitly separate the evaluation of future costs and benefits from the acknowledgment of past investments. Ask: "What are the expected future costs? What are the expected future benefits? Do the benefits justify the costs?" The past investment does not enter this calculation.

Pre-Commit to Exit Criteria

Before starting a project, establish criteria for when to abandon it: "If we have not achieved X by date Y, we will stop." Pre-committing to exit criteria prevents the sunk cost fallacy by making the decision to stop automatic rather than emotional.

Acknowledge the Sunk Cost

Simply acknowledging that the investment is sunk — that it cannot be recovered regardless of what one does — can help. The acknowledgment converts the investment from a factor in the decision (which it should not be) to a fact about the past (which it is).

Seek External Perspectives

External advisors who are not emotionally invested in the project can evaluate it more objectively. An advisor who has no sunk cost in the project can ask the question "is this worth continuing?" without the psychological burden of prior investment.

Contemporary Relevance

The sunk cost fallacy is relevant to several contemporary issues:

Technology projects. Large technology projects — software systems, infrastructure, enterprise implementations — are particularly vulnerable to the sunk cost fallacy. The enormous upfront costs of these projects create strong pressure to continue, even when the projects are clearly failing.

Military engagements. The phrase "we cannot abandon the sacrifices already made" is a classic expression of the sunk cost fallacy in military policy. The lives and resources already lost cannot be recovered, and the question for the future is whether continued engagement will achieve its objectives — not whether the past sacrifices will have been "in vain."

Climate policy. Investments in fossil fuel infrastructure create sunk costs that make the transition to renewable energy more difficult. The companies and countries that have invested heavily in fossil fuel extraction have incentives to continue, even when the environmental and economic case for transition is overwhelming.

How to Apply This

1. Ask the fresh-start question. When deciding whether to continue a project, ask: "If I were starting today, would I choose this?" If not, the sunk cost is driving the decision.

2. Evaluate at the margin. Compare the future costs of continuing with the future benefits. The past is irrelevant to this comparison.

3. Pre-commit to exit criteria. Before starting a project, define what would make you abandon it. This prevents the emotional escalation of commitment.

4. Acknowledge waste without being paralyzed by it. Some investments will be wasted. Acknowledging this is painful but necessary. The alternative — continuing to waste resources to avoid acknowledging prior waste — is worse.

5. Seek external perspectives. People who are not invested in a project can evaluate it more objectively. Seek their counsel when making decisions about whether to continue.

Sources

1. Stanford Encyclopedia of Philosophy, "Sunk Cost Fallacy." Philosophical overview of the fallacy and its implications for rational choice.
2. Stanford Encyclopedia of Philosophy, "Bias and Reasoning." Context for the sunk cost fallacy within the broader study of cognitive bias.
3. Internet Encyclopedia of Philosophy, "Cognitive Bias." Overview of cognitive biases including the sunk cost fallacy.
4. Internet Encyclopedia of Philosophy, "Loss Aversion." Context for the loss aversion mechanism behind the fallacy.

Related Topics

• What is motivated reasoning? — how motivations affect decisions, including the decision to continue failing projects.
• What is belief perseverance? — the persistence of beliefs and commitments.
• What is the backfire effect? — another form of belief persistence.
• What is rationality? — the standard that the sunk cost fallacy violates.