Investor Psychology: The Most Expensive Mistakes

The shared observation of people who last a long time in markets: most losses come not from missing information but from behavior. An investor applying the same strategy with discipline makes more difference than finding a better strategy.

What follows are documented behavioral patterns, and they share one property: they feel perfectly reasonable while you're living them.

Loss Aversion

In practice it looks like this: you sell the winning position early "to lock in profit" and hold the losing one because "it will come back." You end up throwing away what works and collecting what doesn't.

The antidote is a rule: when you open the position, decide where you will be wrong. The decision gets made while the loss is still not an emotional object.

Herding and FOMO

The moment everyone starts talking about a stock is the moment the stock carries the most news — not the most future return. Where the crowd is thickest is usually where the price has already digested the idea.

Anchoring

The price you paid becomes a reference point in your mind. But the market doesn't know your cost basis, and doesn't care.

"I'll sell when it gets back to my cost" ties the decision not to the company's value today but to an accident of your own history. The right question is: would I buy this stock today, at this price, from scratch? If the answer is no, your cost basis is not a reason to hold.

Confirmation Seeking

Once you've settled on an idea, your brain hunts for supporting evidence and discounts the contradicting kind. The moment you're least critical of your biggest position is exactly the moment you should be most critical.

A simple counter-drug: when opening a position, write down the answer to what development would prove me wrong. Reasons written after the fact always acquit their author.

Overconfidence

Two or three good calls produce the feeling of "I've figured this out." In markets, that feeling is usually paid for through position size — and the first wrong call multiplied by the bigger position erases the sum of the earlier right ones.

Recency Bias

Whatever happened in the last three months feels like what will happen in the next three. That is why people are most optimistic at the top and most pessimistic at the bottom — precisely when they should be doing the opposite.

What Actually Works