You bought shares at £40. They're now at £22. You're holding because selling feels like "locking in the loss" — and you tell yourself they'll recover. This is the sunk cost trap: the past purchase price is controlling a future decision it has absolutely no logical claim over.
The £40 is gone. Selling at £22 does not make it more gone. What matters — the only thing that matters — is whether holding at £22 is the best use of that £22 compared to every other option available to you today.
Why the Brain Does This
Loss aversion is the mechanism. Kahneman and Tversky's research showed that losses feel roughly twice as painful as equivalent gains feel good. Losing £100 hurts more than gaining £100 feels good. The brain treats this asymmetrically.
Selling a losing investment makes the loss real — it moves from "unrealised" to "confirmed". The brain interprets this as finalising a painful event. So it delays. It holds. It waits. It adds more. Anything to keep the loss theoretical rather than official.
The sunk cost fallacy layers on top of loss aversion: if you have already spent significant money, time, or effort on something, abandoning it feels like waste. The investment "deserves" to work out. This is emotional reasoning, not financial reasoning — and it costs people enormous amounts of money every year.
The core principle: Sunk costs are irrelevant to every future decision. They cannot be recovered. The only question is: given current reality, does the next unit of investment — money, time, or effort — have a positive expected return from this point forward?
How This Plays Out Financially
Equities and funds
The most common version: holding a stock or fund that has declined because the purchase price feels like a floor. "It'll come back to what I paid" is not an investment thesis — it is a feeling. The asset has no knowledge of what you paid. Recovery to your purchase price is not guaranteed, not likely on any fixed timeline, and is irrelevant to whether holding it is the best use of capital today.
Property
You bought a buy-to-let in 2022 for £280,000. The market has moved against you. The rental yield barely covers the mortgage. You are reluctant to sell because you would "lose money". But capital deployed in an underperforming asset has an opportunity cost — the return you are not earning elsewhere. The sunk cost (the £280,000 purchase) should play no role in the decision to hold or sell. The decision should be based entirely on future expected return relative to alternatives.
Subscription services and memberships
You paid £600 for an annual gym membership in January. By March you've stopped going. You continue not going — because you already paid. But the £600 is gone either way. The decision going forward is simply: does going to the gym next Tuesday have value to you? The past payment is irrelevant to that question.
Businesses and side projects
You have invested £15,000 and two years of evenings into building a product that has twenty users and flat growth. The sunk cost trap says: keep going, you've already put in so much. The correct analysis says: given what I know now, is the expected value of the next £5,000 and six months of effort — in this project versus alternatives — positive? If the honest answer is no, the prior investment should not change the conclusion.
The Fresh Start Test
The cleanest way to interrupt sunk cost thinking is the fresh start test. Ask yourself: If I did not own this, and had the equivalent cash right now, would I buy it at today's price?
If the answer is no — you would not actively choose to purchase this asset, continue this subscription, or start this project today — then the sunk cost is influencing you. You are holding for reasons unrelated to future value.
This test works because it strips away the purchase history entirely. You evaluate the asset in its current state, against its current prospects, as though you were seeing it fresh. If it does not pass that test, no amount of prior investment makes it rational to continue.
- Identify the sunk cost. What have you already spent that cannot be recovered? Name it. Set it aside explicitly.
- State the future options. What are your actual choices from this point forward? (Hold, sell, add more, exit, switch.)
- Apply the fresh start test. With equivalent cash today and no prior history, which option would you choose?
- Write the original thesis. What did you believe when you made the original decision? Does that thesis still hold based on current evidence?
- Decide based on forward returns only. Make the decision that maximises expected value from here, ignoring all past costs.
Sunk Cost vs. Long-Term Conviction
The sunk cost trap has a dangerous doppelganger: genuine long-term conviction. Holding a quality investment through a temporary drawdown is not the same as holding a bad investment because you overpaid. The distinction matters.
The question to ask is: Has the investment thesis changed? If the reasons you bought are still intact and the price has just moved against you temporarily, holding has a rational basis. If the thesis has deteriorated — the business fundamentals have changed, the market dynamic has shifted, the project assumptions were wrong — then holding because of what you paid is a sunk cost trap.
| Situation | Sunk Cost Thinking | Forward-Looking Thinking |
|---|---|---|
| Stock down 35% | "I can't sell now — I need to get back to breakeven" | "Does this company's thesis still hold at today's price?" |
| Unused gym membership | "I need to use it — I already paid for the year" | "Does going today have value? Separate from cost." |
| Failing business | "We've put two years into this — we can't stop now" | "Does the next six months of effort have positive expected value?" |
| Career path | "I've been in this job ten years — too late to switch" | "Does staying provide better returns on the next five years than switching?" |
The Emotional Accounting Problem
Mental accounting is a related bias: people categorise money differently depending on how they got it or what they spent it on, even though money is fungible. The £22 sitting in a fallen stock position is worth exactly as much as £22 in cash — not less. The emotional weight attached to "but I paid £40" is real, but it is not financial reality.
One practical intervention: before reviewing any investment or financial decision, write down the current value only — not the purchase price. Evaluate purely on current state and forward prospects. Then look at the purchase price. Notice whether it changes your reasoning. If it does, the sunk cost trap is active.
Tax and the Sunk Cost Trap
In the UK context, there is one scenario where the past cost is legitimately relevant: tax planning. Crystallising a loss can offset capital gains elsewhere in your portfolio. In this case, the original cost basis matters for the tax calculation — but the holding decision should still be evaluated on forward return alone. Tax efficiency is a consideration layered on top of the core investment decision, not a reason to hold a bad position.
Similarly, bed-and-ISA or bed-and-spouse strategies can transfer assets while resetting the cost basis. If you are holding a losing position partly to avoid "confirming" a loss, and you have taxable gains elsewhere, the tax angle may make realising the loss financially optimal — a case where what feels emotionally costly is mechanically beneficial.
Beyond Money: Careers, Relationships, Degrees
The sunk cost trap extends to every domain where you invest time or effort. A degree you are halfway through and no longer want. A relationship that is no longer working but has years of shared history. A career path you have been on for a decade that no longer fits where you want to go.
The time already spent is identical to money already spent: it is gone either way. What you decide today changes only what happens from today forward. The question is never "how much have I put in" — it is "what is the best use of the time and energy I have from this point."
This is not an argument for impulsive quitting. There are genuine reasons to persist through difficulty. But those reasons should be about future value — what you will gain by continuing — not about not wasting what you have already spent.
The reframe: Stopping is not wasting past effort. Continuing when the expected return is negative — because you have already invested — is wasting future resources. The sunk cost trap inverts what counts as waste.
Building the Habit
Like most cognitive biases, the sunk cost trap is not eliminated by knowing about it. You need a process that interrupts the automatic reasoning. Three habits that help:
- Scheduled reviews with pre-set criteria. When you make an investment or major decision, write down the conditions under which you would exit. A stock: "I will review if it falls 25% from purchase and the thesis has changed." A project: "I will evaluate stopping if we have fewer than 50 users after 12 months." These criteria remove the decision from the moment of loss.
- Decision journals. Record why you made a decision at the time. Reviewing this later separates "the thesis I had then" from "what I know now" — making it easier to see when the original thesis has expired.
- The outside view. Ask: if a friend described this situation to me — same numbers, same position — what would I tell them to do? We consistently give better advice to others than to ourselves on sunk cost decisions, because we are not emotionally attached to their purchase price.
The sunk cost trap is one of the most expensive cognitive biases in personal finance — not because any single decision is catastrophic, but because it operates continuously, across every decision where you have already committed something. Training yourself to consistently separate past cost from future value is one of the highest-return skills in financial life.
For a related framework on financial decision-making, see the decision framework that removes regret.
Frequently Asked Questions
What is the sunk cost trap in personal finance?
The sunk cost trap is the tendency to continue investing money, time, or energy into a failing decision because of what you've already spent — even when future prospects are poor. It happens because humans feel losses more intensely than equivalent gains, making it psychologically painful to "give up" on something you've already paid for.
Why is it so hard to cut financial losses?
Loss aversion — identified by Kahneman and Tversky — makes losses feel roughly twice as painful as equivalent gains feel good. Admitting a loss means accepting that past spending was wasted, which triggers shame and regret. The brain avoids this by convincing you the situation will turn around, even when evidence says otherwise.
What is a sunk cost in investing?
A sunk cost in investing is any money already spent that cannot be recovered — the purchase price of a stock, deposits on a property, or fees already paid. Sunk costs are irrelevant to every future decision. The only question that matters is whether the next pound or dollar you put in has a positive expected return going forward.
How do you overcome the sunk cost fallacy?
Use the "fresh start" test: imagine you did not own this asset and had the equivalent cash today — would you buy it at its current price? If no, sell. Separate the decision from the purchase history entirely. Writing down the original investment thesis and checking whether it still holds also interrupts emotional reasoning.
Does the sunk cost fallacy apply to things other than money?
Yes. The sunk cost trap applies to careers (staying in a job because of years invested), relationships (remaining in harmful situations because of shared history), education (completing a degree you no longer want), and businesses (continuing to fund a failing product). The decision framework is identical: only future value matters, not past cost.