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Trading psychology: why investors underperform their own portfolios
The average investor's returns tend to trail the returns of the very funds they own. The gap is not caused by picking the wrong assets — it is caused by when they buy and sell them. As Benjamin Graham put it, the investor's chief problem, and even his worst enemy, is likely to be himself. Below are the five biases that cost the most, and the habits that reduce each one.
The five biases that cost the most
Loss aversion
- What it is:
- A loss feels considerably worse than an equivalent gain feels good.
- What it costs:
- Winners get sold early to bank the good feeling, losers get held so the loss stays unrealised. The portfolio slowly fills with the positions that are working least.
- What reduces it:
- Decide the exit condition when you buy, in writing, and phrase it about the business — not the price you paid.
Recency bias
- What it is:
- The last few months feel like the permanent state of the world.
- What it costs:
- Risk gets added after a calm run and cut after a drawdown — the exact inversion of what the cycle rewards. Volatility clusters: calm stretches are followed by spikes, not by more calm.
- What reduces it:
- Compare today's conditions to several past cycles, not to last quarter. Set your allocation when nothing is happening, so you are not choosing it under stress.
Overconfidence
- What it is:
- Confidence in a forecast grows faster than the evidence behind it.
- What it costs:
- Position sizes drift past what the thesis justifies, and a single mistake becomes permanent rather than survivable.
- What reduces it:
- Size positions so you could hold them through a 40% drawdown without selling. Conviction you cannot sleep with becomes a forced sale at the worst possible moment.
Herding
- What it is:
- A crowded trade feels safer precisely because it is crowded.
- What it costs:
- You buy the story at the point of maximum agreement, which is usually the point of maximum price. Bull markets mature on optimism and end on euphoria.
- What reduces it:
- Write the bear case before you buy. If you cannot state what a thoughtful seller sees, you have a narrative rather than a thesis.
Anchoring
- What it is:
- Your purchase price, or a past high, becomes the reference point for what the stock is 'worth'.
- What it costs:
- Decisions get made against a number the market has no memory of. A position is held because it 'should' get back to a level, not because the business supports it.
- What reduces it:
- Re-underwrite the position from today's price and today's fundamentals. Ask whether you would buy it now at this price with no history.
Five habits that make behaviour an edge
- 1
Write a one-paragraph thesis
State what the business does, why it should earn more in five years, what would prove you wrong, and what you would accept paying. A written thesis converts vague conviction into something you can test, review and honestly abandon.
- 2
Let position size carry the risk, not willpower
Patience is a function of sizing. If a position is small enough that its worst plausible quarter does not change your life, you can hold it through the volatility that produces the return.
- 3
Separate volatility from risk
Price swings are the fee for long-term returns. The actual danger is permanent impairment of the underlying business. Reacting to the first destroys returns; ignoring the second destroys capital.
- 4
Review on a schedule, not on a headline
Pick a cadence — monthly or quarterly — and review theses then. Decisions made during a news spike are mostly bias with a rationale attached.
- 5
Track your own decisions, not just your returns
Log why you bought and why you sold. Over a year, the pattern of your mistakes becomes visible, which is the only way to stop repeating them.
Why this matters more than stock picking
Most long-term returns arrive in a small number of unpredictable sessions. Being invested is what captures them, which means the decisions that keep you invested matter more than the decisions that fine-tune what you own. Charlie Munger's version of this is blunt: the big money is not in the buying and selling, but in the waiting.
That is also why drawdowns deserve respect rather than panic. Recovering from a 50% decline requires a 100% gain, so avoiding permanent damage — through sizing and diversification across genuine drivers rather than across tickers — does more for compounding than an extra percentage point of upside on any single idea.
None of this requires predicting anything. Howard Marks framed it simply: you can't predict, you can prepare. Preparation is written theses, deliberate sizing, and a review schedule you follow when the market is calm and when it is not.
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