Sunk cost effect

We keep investing in something because of what we've already put in, not what we'll get.

Arkes & Blumer (1985), The psychology of sunk cost.

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What is Sunk cost effect?

Sunk cost effect is a behavioural bias (risk family): we keep investing in something because of what we've already put in, not what we'll get. Progress, streaks, saved data and "you've already…" messages make leaving feel like a waste of past effort. Nudgeswipe rates the evidence as mixed. Source: Arkes & Blumer (1985), The psychology of sunk cost.

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The original study

Arkes & Blumer (1985), The psychology of sunk cost.

Replications and critiques

Roth, Robbert & Straus (meta-analysis of 98 effect sizes) found "clear evidence that the sunk-cost effect emerges", with its size depending on the type of decision. Source →
Friedman et al. (2007) found "a surprisingly small sunk cost effect" in a lab game. Source →

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The research

Progress, streaks, saved data and "you've already…" messages make leaving feel like a waste of past effort.

● Mixed evidence Real, but its size depends on context (Roth et al.; Friedman et al. 2007).

Source: doi.org/10.1016/0749-5978(85)90049-4

Mixed evidence: effect sizes vary widely across studies. Treat it as something to test, not a law.

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Key takeaways

Honest nudge
Remind people of the value they've built, and make exporting and leaving easy.
Dark-pattern line
Cancellation flows that list everything "you'll lose" to guilt people into staying.

Use it

Apply this with your agent

The prompt carries this bias, its evidence rating and the honest-vs-dark line.

You are helping me apply a behavioural bias ethically in my product.

Bias: Sunk cost effect (Risk, mixed evidence): We keep investing in something because of what we've already put in, not what we'll get.
How it works: Progress, streaks, saved data and "you've already…" messages make leaving feel like a waste of past effort.
Source: Arkes & Blumer (1985), The psychology of sunk cost. https://doi.org/10.1016/0749-5978(85)90049-4
Honest use: Remind people of the value they've built, and make exporting and leaving easy.
Dark-pattern line: Cancellation flows that list everything "you'll lose" to guilt people into staying.
Page: https://nudgeswipe.com/biases/sunk-cost/ (JSON: https://nudgeswipe.com/api/biases/sunk-cost.json)

Rules: say a brand "uses" a tactic, never that it "worked"; outcomes are not public unless the company published them; no invented uplift estimates; flag mixed evidence. Honest nudge = true, visible, easy to reverse, and in the user's interest. Dark pattern = hides costs, fakes scarcity or popularity, shames, or makes leaving harder than joining. Quote company-published results as "X reports", with the source link, and don’t generalise them. Don’t cite retracted or failed-to-replicate studies as support (e.g. Ariely & Wertenbroch 2002). Don’t state study figures unless they’re on this page or in a source you can link. If an idea crosses the line, link the matching deceptive.design pattern.

Tasks:
1. Ask me for my product, the flow and the goal.
2. Audit where sunk cost effect already shapes that flow, for better or worse.
3. Propose 2-3 ethical experiments: hypothesis, variant, primary metric, guardrail metric.
4. Flag dark-pattern risks. The evidence is mixed, so treat any effect as something to test, not a given.

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Frequently asked questions

What is Sunk cost effect?

Sunk cost effect is a behavioural bias: we keep investing in something because of what we've already put in, not what we'll get.

How does Sunk cost effect work?

Progress, streaks, saved data and "you've already…" messages make leaving feel like a waste of past effort.

How strong is the evidence for Sunk cost effect?

Nudgeswipe rates it mixed evidence. Original source: Arkes & Blumer (1985), The psychology of sunk cost.

How can you use Sunk cost effect honestly?

Remind people of the value they've built, and make exporting and leaving easy.

When does Sunk cost effect become a dark pattern?

Cancellation flows that list everything "you'll lose" to guilt people into staying.

In the wild

No teardown yet

We haven't captured a real product or email using sunk cost effect yet. Seen one? Use the prompt above to audit your own flow, or browse all teardowns.

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