What behavioural finance actually claims
Not that people are stupid. That people depart from the textbook investor in directions you can predict in advance — which is what makes the departures worth studying rather than merely laughing at.
Chapter 1 · Beginner
Most of this course has taught you machinery: how to discount a cash flow, read a balance sheet, price a bond. This subject asks a different question. Given that the machinery is learnable, why do people who have learned it still do badly?
The baseline being tested
The standard model assumes an investor who assesses the risk and return of the available options and arrives at a portfolio suiting their risk aversion. It is not a claim that people are calculating machines. It is a claim that their errors are random — some too optimistic, some too pessimistic, cancelling out, leaving prices roughly right.
Behavioural finance attacks the cancelling, not the erring.
That is the whole of the disagreement, and it is sharper than "people are irrational". If errors were random, they would wash out in aggregate and you could not profit from knowing about them, nor could you protect yourself by anticipating your own. The behavioural claim is that errors have a direction, that the direction is the same for most people, and that it is therefore predictable in advance.
Why direction matters so much
Consider two worlds.
World A: random error. Half of investors hold their losers too long, half sell them too early. Nothing systematic appears in the data. You cannot know which kind you are, and there is no general advice to give.
World B: directional error. Most investors hold losers and sell winners, in a ratio you can measure. Now there is a fact about human beings, it shows up in millions of accounts, and you can ask whether you do it too.
Chapter 3 shows the data say World B, and says by how much. That single finding tells you the field is making an empirical claim, not a philosophical one.
Working the problem
Your friend's summary misses three things.
It is a claim about systematic direction, not about intelligence. The biases are found in professionals, in high-IQ investors, in people who can state the bias correctly. Knowing a bias exists is weak protection against it — which is itself a finding, and an uncomfortable one for a course like this.
It is measured, not asserted. The claims are made against account-level records of real trades: hundreds of thousands of investors, over years. That is what moves a statement from "people are emotional" to "the proportion of gains realised is 0.148 against 0.098 for losses".
It is specific about mechanism. "Irrational" explains nothing and predicts nothing. "People evaluate outcomes relative to a reference point, and are risk-seeking below it" predicts which trades get held, when, and in which month the pattern reverses. A theory that forbids things is worth more than one that permits everything.
What would make it wrong: if the measured asymmetries disappeared in better data, or appeared only in small unrepresentative samples, or vanished once trading costs and liquidity needs were accounted for. These are live tests, and some claimed effects have not survived them. The honest position is that the central findings in this subject are robust and the periphery is contested — and chapters 9 and 10 will show you where the disputes are rather than hiding them.
What this subject is not
It is not a trading strategy. Knowing that other people hold losers does not tell you what to buy. Every chapter here is about understanding and about your own conduct, not about signals.
It is not an excuse. "I am loss averse" is a description of a tendency, not a reason to act on it. The last chapter is about building arrangements that work even though the tendency is real.
It is not a licence to diagnose other people. The literature's most useful application is to the one portfolio you control.
The point
Behavioural finance does not claim people are stupid; it claims their errors run in the same direction, which means those errors survive aggregation instead of cancelling and can be anticipated. That is an empirical claim, tested against account-level records of real investors, and it is specific enough to be wrong. The value of the subject is not in spotting other people's mistakes but in recognising a documented tendency in yourself and arranging your affairs so it costs you less.
Check yourself
4 questions. Every answer is explained afterwards, including the ones you get right — guessing correctly is not the same as knowing. Score 70% or more and the chapter is marked done.
Question 1 of 4
0 of 4 answered. You can submit with questions unanswered — they simply score zero.
Now do it with your own numbers
A friend says behavioural finance is "just saying people are irrational, which everyone knows". Explain what the field actually claims that this summary misses, and say what would have to be true for the claim to be wrong.
The interesting word is not "irrational". It is "predictably". Ask yourself what a random error would look like in the data, and how that differs from what is found.