6 ideas
Investor biases can be sorted into four root sources: ego (overrating one's own skill and knowledge), conservatism (preferring the status quo and fearing loss), attention (weighting what is vivid or salient over what is statistically representative), and emotion (letting current feelings shape how risk is judged). Diagnosing which root drives a mistake matters more than naming the specific bias, because each root calls for its own countermeasure.
Discretionary judgement fails in markets because biases act in the moment of decision, when they are hardest to notice. Deciding on rules ahead of time, while calm, moves the decision to a point before emotion and ego can interfere. A simple systematic process that is applied every time does better than expert intuition applied case by case.
Feelings do not just sit alongside risk assessment. They change it: when people feel good, risks look smaller and payoffs look bigger, and fear does the reverse. As a result, perceived risk tends to be lowest just as real risk is highest, near market peaks.
Attention goes to what is vivid, recent, and full of story, such as a dramatic crash, a famous success, or a news headline, rather than to base rates and long-run distributions. Investors mistake how easily an example comes to mind for how likely it is, so they overpay for exciting stories and underweight dull but reliable outcomes.
Collecting more information and trading more often make investors more confident without making them more accurate. The gap between confidence and ability then leads to over-trading, too little diversification, and too much concentration. More action feels like more control, but after costs it usually lowers returns.
The difference between what investments earn and what investors actually earn comes mostly from behaviour: buying high, selling in a panic, and chasing performance. On this view, the biggest source of advantage an individual controls is self-management, not picking securities or timing the market. Many people can have good information, but fewer can reliably manage their own behaviour.