5 Behavioural Biases That Can Ruin Your Investing Decisions

Capital at risk. Everything discussed here involves investments that can fall in value, including below what you paid. Understanding your own biases does not make you immune to losses. This is general information, not personalised advice.

Two people can own the identical fund, at the identical cost, over the identical thirty years, and end up with very different amounts of money. Nothing in the fund explains the gap. What explains it is everything that happened between the two of them and the buy button: the month one of them stopped contributing, the week the other sold, the year both of them waited for a better entry point that never announced itself.

This is the uncomfortable centre of passive investing. The strategy is simple enough to fit on a postcard. The hard part is being the sort of person who can follow a postcard for three decades while the news insists you should not.

Below are the five biases that do the most damage to ordinary long-term investors, and — more usefully — the specific structural fixes that work despite them. Not "be disciplined". Systems.

1. Loss aversion

The research literature on loss aversion, associated with Daniel Kahneman and Amos Tversky's prospect theory, finds a persistent asymmetry: losses loom larger in the mind than gains of the same size. A fall of a given amount hurts more than an equivalent rise pleases.

That asymmetry has a specific and expensive consequence. It makes selling during a decline feel like relief rather than like the mistake it usually is. The pain is immediate and real; the cost is abstract and arrives years later, in the form of a recovery you were not present for.

The tell. You find yourself checking the balance far more often when it is falling than when it is rising.

The fix. Decide, in writing and in advance, what would actually make you sell — a change in your goals, your time horizon, or your need for the money. A price fall is not on that list, because a price fall is what equity investing consists of. Write it before you need it, because writing it during a decline is negotiating with yourself while frightened.

2. Recency bias

Recency bias is the tendency to weight the most recent stretch of experience as though it describes the future. After a long rise, further rises feel inevitable. After a sharp fall, further falls feel inevitable. Neither feeling contains information.

The damage is not that it makes you wrong about the direction. It is that it flips your risk appetite exactly out of phase with prices — enthusiastic when things are expensive, cautious when they are cheap.

The tell. Your intended contribution changes depending on what the last few months did.

The fix. Fix the contribution to your income, not to the market. If it went up when you got a pay rise, that is a sound reason. If it went down because of a bad quarter, that is recency wearing a suit.

3. Overconfidence

Overconfidence is the belief that you can identify, in advance, which investments will do better than the market — and it survives contact with evidence remarkably well, because winners are memorable and losers get quietly reclassified as learning experiences.

Its practical signature is trading. More trades, more concentrated positions, more conviction. Every one of those carries a cost — spreads, fees, tax on realised gains — that is certain, in exchange for an advantage that is not.

The tell. You can recite your best trade in detail and cannot immediately name your worst.

The fix. Keep a written record of every active decision, dated, with the reason at the time. Not the outcome — the reason. A year later, read it. This is uncomfortable, which is why almost nobody does it, which is why almost everybody stays overconfident. If you want the evidence on how professional stock selection fares against a plain index, we set it out in index funds vs active funds.

4. Herding

Humans are exceptionally good at reading what other people are doing and copying it, which is a superb survival instinct and a poor investment process. In markets it produces the familiar pattern of enthusiasm concentrating in whatever has already risen most, and abandonment concentrating in whatever has already fallen most.

The modern version is faster and more personal than the old one. A crowd used to be people you knew. Now it is an algorithmically selected feed showing you the small minority of people who did unusually well, which is not a crowd at all — it is a survivorship-biased highlight reel.

The tell. You first heard of an investment less than a month ago and are already considering a meaningful position in it.

The fix. Impose a waiting period on new positions. Two weeks, written down, no exceptions. Most enthusiasms do not survive fourteen days, and the ones that do were probably worth having.

5. Anchoring

Anchoring is over-reliance on the first number you saw. The most common form in investing is the price you paid, which then becomes the level at which the holding is "allowed" to be sold.

The market does not know your purchase price and will never return to it as a courtesy. A holding is worth keeping if you would buy it today at today's price for today's reasons. If you would not, the fact that you paid more is a historical detail, not an argument.

The tell. You are waiting to "get back to even" on something specific.

The fix. Ask the buy-it-today question out loud. If the answer is no, the anchor is doing the deciding, not you. Broad index funds mostly immunise you against this one, because you are not evaluating a company — which is one of the underrated benefits set out in ETFs vs individual stocks.

The bias table, for the fridge door

Notice that not one fix in the right-hand column is "try harder". Every one is a rule made in a calm moment that binds a future version of you who will not be calm. That is the entire technology.

The one system that beats all five at once

Automation. A standing instruction that moves money from your bank into the same broad fund on the same day every month, without asking you.

It defeats loss aversion because there is no sell decision. It defeats recency because the amount does not consult the market. It defeats overconfidence because there is nothing to pick. It defeats herding because the destination was chosen once, in advance. It defeats anchoring because you are buying continuously rather than defending one entry price.

That is not a coincidence — it is why the approach is recommended so persistently. The mechanics of setting it up, and the specific ways people accidentally break it, are in automatic investing: how it works and breaks.

The counter-argument, stated properly

There is a real critique of behavioural finance worth taking seriously: naming a bias is not the same as measuring it, and after the fact you can explain any decision as some bias or other. The framework is dangerously good at producing tidy stories.

That criticism lands. It is also not a reason to ignore the practical part. You do not need the academic apparatus to be settled to notice that you check your balance more in bad weeks, or that you have never once been tempted to buy something that has just fallen 40%. The observation is available to anyone honest with themselves, and the fixes cost nothing and lose nothing if the theory is wrong. An automated monthly contribution into a broad fund is a good idea regardless of what causes people to abandon it.

The weaker claim — that knowing about a bias protects you from it — is the one that genuinely does not hold. Knowing about loss aversion does not make a falling market feel better. Only the rule you wrote beforehand does.

What to check, and what to ask

The honest verdict

Investing well is mostly not an intellectual problem. The information required fits in an afternoon; the behaviour required takes decades. That is an unglamorous conclusion and it is the reason so much financial content ignores it in favour of things that are more interesting to read.

Build the systems while you are calm. Automate the contribution, write the sell rule, keep the journal. Then go and do something else, which is the point of choosing an approach that does not need you — and if you would rather delegate even the discipline, that is genuinely one of the things a robo-advisor sells, examined without enthusiasm in is a robo-advisor worth it.

About this article

This article was produced by Daniel Marchetti — written with AI assistance and reviewed by the NorwegianSpark SA editorial team. YieldNav is operated by NorwegianSpark SA (org. 834 984 172), founded by Thomas Løvås Lokøy and Øyvind. We are not licensed financial advisers, and nothing here is personalised advice. Read our about page and affiliate disclosure.