The disposition effect refers to investors' tendency to sell assets that have increased in value and hold on to assets that have lost value. This effect increases capital gains taxes paid by investors and reduces returns. It underlies patterns in market trading volume and stock price momentum. At an individual level, avoiding this effect reduces losses and taxes paid. At a systemic level, funds exhibiting a strong disposition effect underperform others and are less likely to survive long-term. The disposition effect is driven by prospect theory and loss aversion, as investors are reluctant to realize losses.