The disposition effect is a specific, well-documented pattern in real investor behavior: people disproportionately sell stocks that have gone up in value ("winners") while holding onto stocks that have gone down ("losers") — even when selling the loser would often be more tax-efficient and financially sound. This runs backward from what a purely rational strategy would suggest, which generally favors letting winners run and cutting losses early.
The explanation traces back to mental accounting and loss aversion working together: selling a losing stock forces an investor to formally close the books on a loss, converting a "paper loss" they can still hope will reverse into a real, finalized one — a psychologically painful step that a rational analysis of the numbers alone wouldn't predict investors to avoid so consistently.
Financial researchers analyzing real individual investor portfolios found a consistent, robust pattern: investors sold stocks that had risen in value far more readily than stocks that had fallen, even in situations where selling the losing stock would have produced a tax benefit that made it the clearly better financial move. The pattern held broadly across many portfolios and time periods, not as an occasional quirk but as a systematic bias in how gains and losses get treated differently once they're candidates for sale.