Distinguish sunk costs from future opportunity costs
What you’ve already spent is irrelevant; what you’ll give up going forward is the only cost that matters.
Why it works
Sunk costs are past expenditures that cannot be recovered; they are economically irrelevant to forward-looking decisions. Opportunity costs are future-facing: the value of the best alternative you would pursue if you stopped the current course. Conflating the two produces the sunk cost fallacy — continuing a losing course to avoid "wasting" the prior investment. The discipline is to consciously zero out prior investment and evaluate only the future paths.
How to do it
- When considering whether to continue something, list only what you will gain and give up going forward.
- Explicitly note any prior investment and consciously set it aside from the forward-looking calculation.
- Ask: "If I had not started this, would I choose to begin it now, knowing what I know?" If no, it should probably stop.
- Apply this especially to projects, relationships, and investments that have required large past commitments.
Evidence
The sunk cost fallacy is one of the most robust and replicable findings in behavioral economics. Arkes and Blumer (1985) documented it in controlled experiments, and it has been replicated across many contexts. (rct)
Completely ignoring sunk costs is also not always correct — past investment can be evidence of past commitment or expertise worth preserving. The key is not to let it drive forward decisions.
Sources
- Arkes & Blumer (1985), the psychology of sunk cost, Organizational Behavior and Human Decision Processes
Common mistake
Saying "I’ve put so much into this, I can’t stop now" — which is precisely how the sunk cost fallacy operates. The correct statement is "I’ve put so much into this; what is the best path forward from here?"
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More practices for Opportunity Cost Thinking: What You Give Up When You Choose
- Always name the specific thing you are giving up
When you say yes to something, say explicitly what you are saying no to.
- Convert time decisions to a common currency
Ask "what is my time worth per hour?" and price time commitments in that currency.
- Price the cost of keeping options open
Maintaining optionality is not free — it costs the value you could have captured by committing.
- Maintain an explicit "no" list for categories of commitments
Pre-commit to declining entire categories of requests so each individual yes is forced to clear a higher bar.
- Consider the cost of mediocre vs excellent allocation
Ask not just "is this worthwhile?" but "is this the best use of this resource right now?"
Related concepts
- The Sunk Cost Fallacy: Escaping Bad Investments
Why past investment traps future decisions — and the practices that escape the trap
- Expected Value Thinking: Deciding Under Uncertainty
The math of rational choice under uncertainty, its real limits, and how to use it anyway
- Essentialism, Made Practical
Less but better — the disciplined pursuit of less, trade-offs, and the mechanisms
- Mental Models: Charlie Munger’s Latticework Approach
Building the multi-disciplinary toolkit that lets you see what single-discipline thinkers miss