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Most investment mistakes are not made because of a lack of information. They are made because of the way the human brain is wired to process risk, reward, and uncertainty — patterns that served us well on the savannah and serve us poorly in markets.
Where the biases come from
In 1979 Daniel Kahneman and Amos Tversky published prospect theory, a description of how people actually choose under risk rather than how a textbook says they should. Two of its findings carry most of the weight for investors.
First, people evaluate outcomes as gains and losses from a reference point, not as levels of total wealth. The reference point is usually whatever they started with — the purchase price, last year’s portfolio value, the recent peak.
Second, the value function is steeper for losses than for gains. A loss hurts more than an equivalent gain pleases, so people will take risk to avoid making a loss real that they would never take to secure a gain. This asymmetry, later named loss aversion, is the root from which most of the biases below grow. It is not a flaw in intelligence. It is the default setting of the machinery.
The biases that cost investors the most
Loss aversion. Because a loss weighs more heavily than a gain of the same size, investors hold a losing position to avoid crystallising the loss, and sell a winning one to lock the gain in before it can disappear. Both are the exact opposite of disciplined portfolio management, which asks you to keep what is working and cut what is not.
Recency bias. We overweight recent events when forecasting the future — buying after a rally because it feels safe, and selling after a decline because it feels inevitable. Both are usually the wrong decision at the wrong time. The most recent three years of returns are the least representative sample an investor could choose, and the one they most reliably use.
Herding. It is psychologically comfortable to do what everyone else is doing. It is also how bubbles form and how disciplined investors end up buying at the top and selling at the bottom, in step with the crowd. The crowd is not wrong because it is a crowd. It is dangerous because it removes the friction that would otherwise make you check.
Why do investors sell winners and hold losers?
The pattern has a name. Hersh Shefrin and Meir Statman called it the disposition effect in 1985 — the disposition to sell winners too early and ride losers too long — and traced it to loss aversion, regret, mental accounting and weak self-control working together.
Terrance Odean then measured it. Analysing the trading records of 10,000 accounts at a US discount brokerage from 1987 to 1993, he found that a stock that was up was more than 50 percent more likely to be sold on any given day than a stock that was down (1998). The behaviour could not be explained by rebalancing or trading costs. Worse, the winners investors sold went on to outperform the losers they kept. The only month the pattern reversed was December, when tax-loss selling briefly overrode instinct.
Investors, in other words, were not managing risk. They were managing how a realised loss would feel. The portfolio that results is the residue of that feeling: the positions that worked have been sold, and the positions that did not are still there, waiting to get back to even.
Anchoring and overconfidence
Anchoring. In their 1974 paper on heuristics and biases, Tversky and Kahneman showed that people estimate unknown quantities by starting from an initial value and adjusting from it — and that the adjustment is typically too small, even when the starting value is obviously arbitrary. In a portfolio the anchor is almost always the purchase price, or the index level at which you first paid attention. Neither is information about the future.
Overconfidence. Brad Barber and Odean examined 66,465 US households over 1991 to 1996 and found that the most active traders earned 11.4 percent a year while the market returned 17.9 percent (2000). The average household turned over three-quarters of its portfolio annually. Their conclusion was that overconfidence explains the trading, and the trading explains the shortfall. Conviction is not evidence; it is a feeling about evidence.
The Indian version of the same wiring
None of this is a Western peculiarity. In 2024 SEBI published a study of individual traders in equity futures and options and found that 93 percent lost money between FY22 and FY24, with aggregate losses exceeding ₹1.8 lakh crore over the three years.
That figure is overconfidence, herding and loss-chasing in one number. Each participant believed they were the exception. Each was drawn in by a visible crowd of others doing the same. And stopping after a loss is the one thing loss aversion forbids, because stopping makes the loss final.
The Indian market has also added a large cohort of first-time investors in a period that, taken as a whole, rewarded them. That is precisely the environment in which recency bias is most expensive: the lesson learned is that dips recover quickly, and the portfolio is built on that lesson rather than on a plan. An investor whose entire experience is one phase of a cycle has not been tested by the market yet. They have been flattered by it.
The frameworks that counter them
Behavioural discipline is not about eliminating emotion — that is neither possible nor desirable. It is about building structures that make good decisions the default, even when emotion is loud:
- Written investment policy statements that define, in advance, how you will respond to volatility — so the decision is made before the panic arrives. A rule written in a calm market is the only thing that will hold in a violent one.
- Systematic rebalancing that forces you to sell strength and buy weakness on a schedule, rather than on impulse. It converts loss aversion’s favourite trade into a mechanical one nobody has to feel.
- Pre-committed allocation ranges that make deviation from your plan visible and deliberate, not accidental. Drift within a band is normal; drift outside it is a decision someone has to sign.
- Automated contributions. The systematic investment plan is, at bottom, a behavioural device: it removes the timing decision from a person who would otherwise anchor on the last high and wait.
Discipline is a design problem
Behavioural research keeps making a point that investors resist: knowing about a bias does not remove it. The investors in these studies were not ignorant of loss aversion. They were subject to it anyway, because it operates before reasoning starts.
The practical response is not to try harder. It is to design the process so that the moments where bias does its damage — the sale, the panic, the top-up after a rally — are handled by rules written in advance, and reviewed on a schedule rather than on a feeling.
This is also the honest case for an adviser. The value is not superior prediction. It is a second decision-maker who is not anchored to your purchase price, does not share your recent experience of the market, and is contractually obliged to hold you to the plan you wrote when you were calm. A good process makes the bias visible before it becomes a trade. A good adviser makes sure somebody is looking.
At INVESTSIGHT CAPITAL, this is why we treat behavioural intelligence as equal in weight to investment intelligence. The best research is worthless if psychology undermines its execution.
