The tendency to treat money differently depending on its source, purpose, or label — creating irrational distinctions between units of currency that are economically identical.
“Money is money. A dollar in your pocket is worth the same whether you earned it, won it, or found it — but people don't behave that way.”
— Richard Thaler
Deeper Explanation
Richard Thaler developed mental accounting as a framework for explaining why people treat money differently depending on how they categorise it — despite the fact that a pound is a pound regardless of its source or label. Mental accounting divides money into separate "accounts" in the mind, each governed by different rules, even when the rational decision would treat all funds as fungible. Several specific mental accounting distortions affect investor behaviour. The "house money" effect: gains from investments feel less "real" than the original capital invested, causing investors to take more risk with gains than they would with equivalent amounts of their original capital. Having already "won" the house's money, gamblers and investors feel less inhibited about risking it — even though the money is equally real regardless of its source. The segregation of gains and losses: people prefer to receive good news and bad news separately — "good news segregation" causes investors to want each piece of good news treated as a distinct positive event, while "bad news integration" causes investors to prefer that multiple pieces of bad news be delivered together (to be treated as one aggregate loss, which is less painful than multiple separate losses). This is psychologically comfortable but distorts portfolio reporting and decision-making. The sunk cost fallacy is mental accounting's most famous manifestation: continuing to invest in a project or position because of the money already spent, even when forward-looking analysis clearly argues for exit. The "sunk cost" is in a separate mental account that cannot be recovered, creating a psychological obligation to keep investing that has no rational basis. Thaler's practical remedy: treat all funds as fungible. "The money I made on this stock" is identical to "the money I originally invested." "The money I've lost so far" is irrelevant to the question of whether to hold or exit today.
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