From a recent Washington Post article:

Recent studies have shown that people don’t tend to be rational economic actors; their decisions are based in part on their reactions to the facts at hand. Here’s a chance to test yourself.

1. The Beer Experiment

Imagine you are sitting on a hot beach. Your companion offers to go get a couple of bottles of cold beer. He asks you:

A. What’s the most you’ll pay if the beer can be bought at a local dive?
B. How much if he has to get it at the fancy resort down the street?

When University of Chicago economist Richard Thaler put these questions to two groups several years ago, the average responses were $1.50 for the beer from the dive and $2.65 for the one from the hotel. In economic terms, that makes no sense: If you’re willing to pay $2.65 for a beer, why should you care where it is bought or how much profit the seller makes? But most people do care. They don’t want to be taken advantage of by the dive owners, whose expenses are minimal. But they’re willing to pay more at a resort, where they understand expenses are high.

2. The Vaccine Experiment

Imagine you are public health director in a poor country where a fatal infectious disease threatens a small village of 600 people. You can inoculate every resident with only one of two vaccines. Which vaccine would you choose?

A. The one that will save 200 lives.
B. The one that will result in 400 deaths.

When psychologists Amos Tversky and Daniel Kahneman administered a somewhat more complex version of this test, more than three people chose A for every one who chose B. In fact, the vaccines are exactly equal in their effectiveness, but when facing tough decisions, people feel more comfortable when choices are framed positively (saving lives) rather than negatively (causing deaths).

3. The Gift Certificate Experiment

Imagine you are offered a choice of gifts for participating in a survey. Which would you choose?

A. $85 in cash.
B. A gift certificate for an $80 massage at a local spa.

When Stanford University psychologist Itamar Simonson offered a group of women these choices, a third chose the gift certificate. The rational choice is the cash even for someone who wants the massage. The $85 would cover it while still offering flexibility. But Simonson reasons that women who wanted the massage didn’t trust themselves to take the cash, knowing that guilt would lead them to spend it on something less indulgent.

4. The Wine Experiment

Imagine you bought a case of fine Bordeaux wine at $20 a bottle a decade ago. Now the wine sells for $75 a bottle. What will it cost you to now drink a bottle of wine?

A. $0
B. $20
C. $20 plus 10 years interest
D. $75
E. It’s a savings of $55

Thaler found more than half of all respondents answered zero or a $55 saving. Strictly speaking, the right answer is D, reflecting the opportunity cost of not selling the bottle on the open market. Most people however, focus on the amount already shelled out, the sunk cost.

5. The Ultimatum Game

Imagine you are given $10 and told you must offer to split it with a stranger. The only hitch is that if the stranger rejects your offer, neither of you get anything. How much must you offer?

A. $9
B. $5
C. $1
D. Zero

The rational (that is, selfish) strategy would be to offer $1, which the stranger would be silly to reject. After all, $1 is better than nothing and $9 is the best outcome for you. But in countless repetitions of this ultimatum game, the majority of all offers were between $3 and $5, while anything below $2 was usually rejected by the stranger. The results suggest that people’s desire for money is tempered by notions of fairness.

What These Experiments Mean for Traders

Each experiment exposes a specific cognitive failure that maps directly to a trading error.

The beer experiment demonstrates context-dependence. The rational value of the beer is identical regardless of where it is purchased, just as the rational value of an entry signal is identical regardless of whether the market has been rising or falling. A trader who demands a cheaper entry in a bear market than in a bull market, even when the price behavior meets the exact same entry criteria, is making the beer buyer’s error. The system’s rules apply regardless of the context the trader has placed around the trade.

The vaccine experiment demonstrates framing effects. A stop loss that exits a position at a $500 loss feels different from an exit that “saves $9,500 of remaining capital,” even though both describe the identical action. Traders who cannot take stop losses because the loss framing is psychologically intolerable are the three-to-one majority who chose vaccine A. The stop loss is vaccine B. Both outcomes are identical. The framing is the only difference.

The gift certificate experiment demonstrates self-distrust and commitment devices. The woman who chose the massage certificate over the $85 cash did not trust herself to make the rational choice in a future moment. Systematic trading rules are the trader’s version of the gift certificate: a commitment device built when the mind is clear that prevents the emotional response in the future moment from overriding the correct decision.

The wine experiment demonstrates the sunk cost fallacy. The trader who holds a losing position because they paid $20 for it is drinking the wine for free. The position’s current market price, not the entry price, is the opportunity cost. What was paid is irrelevant to what should be done now. The stop loss forces this accounting correctly by making the exit decision based on current price rather than on the distance from entry.

The ultimatum game demonstrates that fairness considerations override rational self-interest. In trading, this manifests as the reluctance to short a falling stock because it feels unfair to profit from someone else’s loss, or the refusal to add to a position at a higher price because it feels like paying too much. The market does not operate on fairness norms. The systematic approach does not incorporate them. Rules based on price movement produce the rational outcome that the emotional response to fairness considerations prevents.

Frequently Asked Questions

What is the sunk cost fallacy and how does it affect trading?

The sunk cost fallacy is the tendency to factor already-spent costs into current decisions even though those costs are irretrievable regardless of what is done now. In trading, the entry price is a sunk cost. The correct question at any moment is: given current price and current market conditions, what does the system say to do? The entry price is irrelevant to that question. A trader who holds a losing position because they paid $20 for it and don’t want to “lock in” the loss is making the wine drinker’s error.

How does framing affect trading decisions and how can it be countered?

Framing changes the psychological experience of identical outcomes by describing them in positive or negative terms. A stop loss that frames as “taking a $500 loss” feels worse than one that frames as “protecting $9,500 of capital,” even though both describe the identical action. Systematic rules counter framing effects by defining exit criteria objectively, in terms of price levels and percentages, rather than in terms of the loss amount relative to entry.

What does the ultimatum game reveal about market participants?

That fairness norms influence financial decisions even when they produce irrational outcomes. Participants rejected offers that were economically beneficial because the offers felt unfair. In markets, this produces behavior like refusing to short declining stocks, refusing to take profits from positions that others are losing money on, and anchoring exit decisions to entry prices rather than to current market conditions. These norms are real and powerful but have no place in systematic price-based decision-making.

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