Post-mortems of bad investments reveal the cause is never a calculation error but always a psychological bias or emotional trap. Sequoia catalogs ~40 of these, including failing to separate the emotional 'thrill of the chase' from the clinical, objective assessment required for sound decision-making.

Investment gains often come from "multiple expansion," where the market's perception of a business improves, causing it to trade at a higher valuation. This sentiment shift is frequently more impactful than pure earnings growth, and underestimating it is a primary reason for selling winning stocks too early.

Private equity managers often get psychologically anchored to their purchase price. Instead of cutting losses on a poorly performing asset to redeploy time and capital, they hold on in the vain hope of getting their money back, turning a bad deal into a time-consuming, mediocre one.

To avoid emotional, performance-chasing mistakes, write down your selling criteria in advance and intentionally exclude recent performance from the list. This forces a focus on more rational reasons, such as a broken investment thesis, manager changes, excessive fees, or shifting personal goals, thereby preventing reactionary decisions based on market noise.

Kahneman's research reveals a critical asymmetry: we prefer a sure gain over a probable larger one, but we'll accept a probable larger loss to avoid a sure smaller one. This explains why investors often sell winning stocks too early ("locking in gains") and hold onto losing stocks for too long ("hoping to get back to even").

A study in the book "Art of Execution" found the world's best investors have a win rate equivalent to a coin flip on their top 10 ideas. This proves superior returns come from how positions are managed after the initial buy decision, not from superior stock picking alone.

Short-term performance pressure forces fund managers to sell underperforming stocks, creating a self-fulfilling prophecy of price declines. Investors with permanent capital have a structural advantage, as they can hold through this volatility and even buy into the weakness created by others' behavioral constraints.

A Wall Street Journal experiment pitted a monkey throwing darts at a stock list against professional traders. Over a ten-year span, the monkey's long-term, passive 'buy-and-hold' strategy won. This demonstrates the power of long-term investing over short-term, active trading.

Even sophisticated institutional investors exhibit significant behavioral biases. Research on their trades revealed that while their buying decisions added value, their selling decisions were so poor that a random selling strategy would have outperformed their actual sales by 100-200 basis points. They seem to apply discipline to buying but not selling.