What has already been spent cannot be recovered by spending more

Decisions only run forwards

Every choice about money is a choice between futures. Whatever has already been paid is identical in all of them, which means it cannot distinguish between them and therefore cannot inform the decision. Economists call such an amount a sunk cost, and the rule is that sunk costs are irrelevant to what happens next.

The rule is easy to state and unusually hard to apply, because it contradicts something that feels close to a moral principle: that waste is bad and that abandoning something already paid for is wasteful. The waste, though, has already occurred. Continuing does not undo it; it only adds to the total.

That is the whole of the argument. Everything else is about why it is so difficult to act on.

Why the grip is so strong

Several things reinforce each other here. There is loss aversion — walking away means writing the amount off, which converts an ambiguous situation into a definite loss. There is a preference for consistency, since abandoning a commitment involves publicly, or privately, revising a previous judgement.

There is also a genuine aversion to waste that is usually sensible and misfires here. Finishing something you paid for feels like extracting value; in reality the money left regardless, and the only thing being extracted is whatever the thing is actually worth from this point forward.

Notice that none of these operate on the arithmetic. Someone in the grip of a sunk cost can generally state the correct principle if asked in the abstract, which is a reliable sign that the difficulty lies elsewhere.

Where it costs households money

A car with a history of expensive repairs is the standard example, and it earns its place. Each new fault is assessed against everything already spent — having put so much in, it seems wrong to stop now — when the real comparison is between the cost of this repair and the cost of the alternative. The previous bills are gone under either choice.

It appears in investments as the belief that the price paid matters to whether something should be held. It does not. What a holding cost is a fact about the past; whether it is worth holding depends on what it is expected to do from here, and the two questions have no connection despite feeling closely related.

It appears in half-finished projects, in courses paid for and not enjoyed, in memberships that continue because a joining fee was paid, and in renovations where each additional stage is justified by the ones already completed. The pattern is identical in all of them.

The question that dissolves it

One reframing does most of the work: if I were starting today, with none of this behind me, would I choose to spend this amount for this outcome? It works because it removes the history from the comparison, which is precisely the thing that should not have been in it.

Answering honestly is harder than asking, and there is a second version that helps when the first is uncomfortable. Imagine the decision belongs to somebody else who has spent nothing. Advice given to a stranger is generally cleaner than advice given to oneself, for the straightforward reason that the stranger has no history to defend.

Both questions push towards the same place: comparing the remaining cost with the remaining benefit, and ignoring everything before this moment.

Where persistence is not a fallacy

It would be a mistake to treat every continuation as an error. Some commitments carry real forward-looking value that has nothing to do with what was already spent — a contract with a penalty for exit, a course where the qualification only arrives at the end, a project where the remaining work is genuinely small relative to what it unlocks.

Those are legitimate reasons to continue, and the test is whether the reason points forwards. If the argument for carrying on can be made without mentioning what has already been spent, it is a real argument. If removing that fact collapses it, it was a sunk cost dressed up.

The general principle is robust; what to do about a specific commitment is not, since it depends on the terms, the amounts and what else the money could be doing. Where the sums are significant, a regulated adviser can weigh the forward comparison with the whole picture in view.