The Reference Point Problem
When you buy a stock at £100 and it falls to £60, you experience a £40 loss — even if all available evidence suggests that £60 is exactly the right price for what the company is worth today. The purchase price has become your reference point, and your evaluation of the investment is anchored to it. This creates a fundamental problem: you are looking backward at a cost you have already incurred, when the only relevant question is what the investment is worth now and what it might be worth in the future.
The sunk cost fallacy is closely related. The £40 you are "down" on the investment is gone regardless of what you do next. Holding the investment when you would otherwise sell it — purely to avoid realising the loss — is keeping money in an investment for reasons that have nothing to do with its future prospects. The question to ask is not "am I up or down?" but "is this the best place for this money going forward?"
Mental Accounting
Richard Thaler developed the concept of mental accounting to describe how people treat money differently depending on its origin, location, or intended purpose — even when the money is, economically speaking, perfectly fungible. A pound is a pound regardless of where it came from or which account it sits in. But people do not behave this way.
The "house money" effect is a classic example: investors who have accumulated gains in a portfolio tend to take more risk with those gains than they would with their original capital. The profits feel like "the house's money" — free money that does not really count — even though it is unambiguously their own wealth. The reverse also holds: investors guard their original principal with far more caution than financial logic would justify.
Mental Accounting Puzzles
Mental accounting explains several financial puzzles that are otherwise hard to understand. Many people simultaneously hold high-interest credit card debt and maintain a low-interest savings account — a straightforward loss of money. But the savings account is in the "emergency fund" mental account and feels different from the debt account. Paying off the debt with the savings would be rational, but it would empty the safety net, which feels psychologically dangerous.
Thaler also notes that people tend to overspend a tax refund because it feels like a windfall rather than their own earned income — even though it is simply their own money that was withheld and returned. The source shapes the behaviour.
The Practical Fix
The corrective is to evaluate every investment position purely on its forward-looking merits. The relevant question is: "Given what I know today, is this investment the best use of this capital?" If the answer is no, the fact that you are sitting on a loss is irrelevant — and continuing to hold it is an expensive emotional decision masquerading as an investment decision. Professional portfolio managers adopt a "would I buy this today?" test: if you would not buy the position at today's price with fresh capital, you have no rational reason to hold it.