Sunk cost
Definition
A sunk cost is a past expenditure of money, time, or effort that cannot be recovered regardless of future choice. The economic principle — that sunk costs should not influence forward-looking decisions — is widely understood in theory and widely violated in practice, producing the well-documented "sunk cost fallacy."
Why it matters
The sunk cost concept matters because the everyday human tendency is to honor past investment when deciding the future — "I've put too much in to walk away now." Economists call this the sunk cost fallacy and consider it irrational: the past money is gone whether you continue or stop, so the only relevant question is the expected return on remaining effort. Behaviorally, however, the fallacy is the most common error in personal investment decisions, business decisions, and relationship decisions. Recognizing it is one of the highest-leverage skills in personal decision-making.
Origin and lineage
The technical concept of sunk cost is foundational in microeconomics, with the principle of "ignore sunk costs" articulated in early 20th-century economic writing. Hal Arkes and Catherine Blumer's 1985 paper "The psychology of sunk cost" in Organizational Behavior and Human Decision Processes brought the concept into experimental psychology, demonstrating that ordinary people consistently honor sunk costs even when explicitly told the prior expenditure was unrecoverable. Their classic experiment showed that students who had paid for a movie ticket were more likely to brave a snowstorm to attend than students who had been given the ticket free.
Research evidence
The sunk cost effect has been replicated in domains including business investment (escalation of commitment to failing projects), personal relationships, governmental project continuation, and military strategy. Children below a certain developmental threshold do not show the effect, suggesting it is learned through cultural transmission rather than evolved as a default. Some animal species show analogous behavior, complicating the developmental story. The neural correlates of sunk cost reasoning have been mapped in fMRI studies, with activation in the dorsal anterior cingulate cortex during sunk-cost decisions.
Common misconceptions
"Ignore sunk costs" is a normative principle, not a description of how people think. Most decision support advice that quotes the principle as if everyone already understands it underestimates how strongly the contrary intuition operates. The concept also is sometimes mistaken for "never reconsider past commitments." It says only that the size of past investment should not, by itself, drive whether to continue. Genuine reasons to continue may exist; the past investment is just not one of them.
How LifeByLogic measures it
Sunk cost is a key consideration in the Should I Quit framework, where it appears as one of the variables in the structured decision review. The Cognitive Bias Susceptibility tool also measures sunk-cost susceptibility using the validated task structure from the Adult Decision-Making Competence battery (Bruine de Bruin et al., 2007).