Loss aversion and prospect theory

Most trading advice assumes you will do what you decided to do. You will cut the loser at the stop, let the winner run to the target, and size every trade the same. The evidence from psychology says that, left to your own devices, you will do roughly the opposite. This module is about why. Not to make you feel broken, but because a bias you can name is one you can build a rule around.

We start with the most important one.

The experiment that changed economics

In 1979 Daniel Kahneman and Amos Tversky published "Prospect Theory: An Analysis of Decision under Risk." They asked people simple questions about gambles and found that the answers did not match the standard economic model, in which a rational person values money in absolute terms and weighs outcomes by their probability.

Three findings matter for traders.

1. People evaluate outcomes relative to a reference point, not in absolute wealth. A trader with $50,000 who was at $52,000 an hour ago does not feel like someone with $50,000. They feel like someone who lost $2,000. The reference point is usually the recent peak, the entry price, or the number you told yourself you would make today.

2. Losses hurt more than equivalent gains feel good. Kahneman and Tversky's value function is steeper on the loss side than the gain side. In later work with a larger sample, they estimated the ratio at roughly two to one: a loss of a given size feels about twice as intense as a gain of the same size. The exact number varies by person and study, but the asymmetry is one of the most replicated results in the field. This is what "loss aversion" means.

3. People are risk-averse for gains and risk-seeking for losses. Offered a sure $900 or a 90% chance of $1,000, most people take the sure thing. Offered a sure loss of $900 or a 90% chance of losing $1,000 (with a 10% chance of losing nothing), most people take the gamble. Same expected value, opposite preference, purely because one is framed as a gain and the other as a loss.

Key idea: You are not neutral about money. Losses feel roughly twice as large as gains, and once you are losing, your brain becomes willing to gamble to get back to even. Every rule in this course exists because of that asymmetry.

What this looks like on the screen

Take finding three and put it in a trade.

You are long a futures contract with a stop 10 ticks below entry and a target 20 ticks above. Price moves 7 ticks against you. You are now in the "loss" region of the value function, where people become risk-seeking. The thought arrives, fully formed and reasonable-sounding: "It's about to bounce, I'll give it a few more ticks." You move the stop. Nothing in the market changed; the bounce is no more likely than it was a minute ago. What changed is that you are now choosing between a sure loss (take the stop) and a gamble (hold and maybe get back to even), and prospect theory says you will take the gamble.

Now the same trade goes 12 ticks in your favour. You are in the "gain" region, where people are risk-averse. The thought arrives: "Lock it in before it reverses." You close at +12 instead of +20. Again, nothing changed in the market.

Repeat this across a year and your average loss is larger than planned and your average win is smaller than planned. The system that had positive expectancy on paper is now losing money, and it is not the system's fault.

Loss aversion is not a character flaw

It helps to know that this is not stupidity or weakness. Loss aversion shows up in children, in other primates, and across cultures. There are good evolutionary reasons for an organism to weigh threats more heavily than opportunities. Your brain is doing what it was built to do; it was simply not built for a screen with a flashing P&L.

That framing matters because the alternative, "I just need to be more disciplined," has a poor track record. Willpower is a resource that runs out, usually at the exact moment you need it. What works is arranging things so the biased decision never gets made in real time: the stop is placed at entry and not touched, the target is set before the trade, and the size is fixed by a rule, not by how the last trade felt. Module 3 is entirely about building that machinery.

The reference point trap

The subtlest part of prospect theory is the reference point, because you can move it without noticing.

A trader is up $800 by 10:30. At 11:15 they are up $300. Objectively this is a $300 winning day. Subjectively it is a $500 loss, because the reference point silently became $800. Now they are in the loss region, risk-seeking, and the next trade is oversized to "get back" to a number that was never theirs to keep.

A useful defence is to fix the reference point on purpose. Your daily reference is zero at the open. Your trade reference is the entry, and once the stop is placed, the only two outcomes you are allowed to think about are the stop and the target. Anything else is noise.

Try it: Open your trade log and find the last twenty closed trades. For each one, write down the planned stop distance and the actual loss (for losers), and the planned target and the actual gain (for winners). Compute the average of each. If actual losses are larger than planned and actual wins are smaller, you have just measured your own loss aversion in dollars.

Recap

• Kahneman and Tversky's prospect theory: people judge outcomes relative to a reference point, feel losses about twice as strongly as gains, and become risk-seeking when losing.
• On a trading screen this shows up as moved stops (gambling to avoid a sure loss) and early exits (locking in a sure gain).
• The result is average losses bigger than planned and average wins smaller than planned, which can turn a profitable system into a losing one.
• Loss aversion is universal and biological; the fix is pre-committed rules, not more willpower.
• Fix your reference points deliberately: zero at the open, entry for each trade, and only the stop and target as valid outcomes.