Treat money as fungible across the buckets

A dollar is a dollar no matter which mental account it sits in — decide accordingly.

Why it works

We code money into separate mental accounts and apply different rules to each, violating fungibility — the principle that money is interchangeable. The bucket the money lives in, not the actual choice in front of you, ends up driving the decision. Asking "would I make this same call if the money came from a different account?" exposes the distortion.

How to do it

1. When a money decision feels obvious, name which mental bucket it is drawing from.
2. Re-ask the decision as if the same amount came from a different account (savings vs windfall).
3. If your answer changes, the bucket — not the merits — is driving you.

Evidence

Mental accounting is a well-documented behavioral-economics phenomenon. Thaler’s body of work shows people systematically violate fungibility, treating identical money differently by source and category, in ways standard economic theory cannot explain. (observational)

Much of the evidence is from surveys, choice experiments, and field observation rather than randomized trials; the pattern is robust, individual magnitude varies.

Sources

• Thaler (1999), "Mental Accounting Matters", J. Behavioral Decision Making
• Thaler (1985), "Mental Accounting and Consumer Choice", Marketing Science

Common mistake

Assuming you are too rational for this. Almost everyone treats a tax refund or a bonus as more spendable than identical money already in their account.