Duolingo: sunk cost effect example to copy

What to steal

When you invite people back, name the work they have already finished.
Show how to reopen it, and don’t threaten to wipe it.

What to test next: “Get access to all the units you’ve already completed” vs “Review any unit”. Measure how often people open an old unit.

In plain words

Why it works

Duolingo uses the sunk cost effect: people keep investing because of what they have already put in, not only what they will get next (Arkes & Blumer 1985). The headline is set in lowercase and reads “practice hack: how to review previous lessons on duolingo”. The line under it reads “Get access to all the units you’ve already completed. Here’s how!” The post is dated “OCTOBER 1, 2026” and credited “DUOLINGO TEAM”. It points at units already finished and says how to open them again. It does not say the units will be lost. Evidence for the effect is mixed. Whether this line brings people back to old units is not public.

Honest nudge or dark pattern?

Honest nudge

Honest nudge: the line describes access to work already done, and it does not list what you lose if you stop.

Sources: Captured by Nudgeswipe, 4 Nov 2026 (blog post, logged out)

Context

Duolingo blog post, logged out, on 4 Nov 2026. The date printed on the post is 1 October 2026. The picture under the byline is the owl lifting a barbell. This teardown does not treat the drawing as a claim.

The research

The research behind it

● Mixed evidence Why mixed: Real, but its size depends on context (Roth et al.; Friedman et al. 2007). · More on sunk cost effect, with sources →

Next: run it on your product

Try it on your own product

Opens your assistant with this teardown, our verdict and the steps above. It asks about your product first, then suggests tests you can run.

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You are helping me apply a behavioural-design pattern ethically.

Pattern: Duolingo: Duolingo sends you back to units you've already completed
Bias: Sunk cost effect: We keep investing in something because of what we've already put in, not what we'll get.
Why it nudges: Duolingo uses the sunk cost effect: people keep investing because of what they have already put in, not only what they will get next (Arkes & Blumer 1985). The headline is set in lowercase and reads "practice hack: how to review previous lessons on duolingo". The line under it reads "Get access to all the units you’ve already completed. Here’s how!" The post is dated "OCTOBER 1, 2026" and credited "DUOLINGO TEAM". It points at units already finished and says how to open them again. It does not say the units will be lost. Evidence for the effect is mixed. Whether this line brings people back to old units is not public.
Our verdict: Honest nudge. Honest nudge: the line describes access to work already done, and it does not list what you lose if you stop.
Source page: https://nudgeswipe.com/examples/duolingo-units-already-completed/ (JSON: https://nudgeswipe.com/api/teardowns/duolingo-units-already-completed.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 (URL or screenshots) and the goal.
2. Audit where this pattern already appears, or could, in that flow.
3. Propose 2-3 ethical experiments: hypothesis, variant copy or UI, primary metric, and a guardrail metric (refunds, complaints, cancellations).
4. Flag every dark-pattern risk and say how to stay on the honest side.

Grok Cursor

Grok and Cursor open with the prompt filled in. Gemini and Codex can’t take a prefilled link, so the prompt is copied and you paste it in.

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Sources

Our capture of blog.duolingo.com, 4 Nov 2026
Arkes & Blumer (1985), The psychology of sunk cost. doi.org/10.1016/0749-5978(85)90049-4
Captured by Nudgeswipe, 4 Nov 2026 (blog post, logged out): blog.duolingo.com

Duolingo sends you back to units you’ve already completed

5 new nudges, every Friday

Why it works

Honest nudge

The research behind it

Try it on your own product

5 new nudges, every Friday