The Myth of the Rational Investor
Economics has a favorite character. The Rational Man. He always knows what he wants. He always picks the best option. He never panics, never gets confused, never makes a dumb choice because he’s tired or emotional.
I spent 20 years teaching science before switching to IT. And if there is one thing both fields taught me, it is this: models are useful until they meet real people. The Rational Man is a beautiful model. But he doesn’t exist.
Chapter 8 of Burton and Shah’s book is where they lay this out. And it is the setup for everything that comes next in behavioral finance.
How Economists Think You Choose Things
Let’s start simple. Imagine you have $100. You can buy guns or butter. (Classic economics example. Don’t ask why it’s always guns and butter. It just is.)
Guns cost $5 each. Butter costs $2 per tub. So you could buy 20 guns, or 50 tubs of butter, or some mix of both.
Economists say you will pick the combination that makes you the happiest. They call this happiness “utility.” Every possible combination of guns and butter gets a happiness score. You pick the combo with the highest score.
For this to work, economists need three rules:
- Every option must have a score. You can’t say “I have no idea how I feel about 7 guns and 33 tubs of butter.” Everything gets ranked.
- You can always compare two options. Option A is better than B, or B is better than A, or they’re equal. No “I can’t decide” allowed.
- Your rankings must be consistent. If you prefer A over B, and B over C, then you must prefer A over C. No circular preferences.
These rules sound reasonable. And when you’re choosing between things you can see and touch, they mostly work. You go to a store, you look at the prices, you pick what you want. Simple.
But here’s the thing. Life is not a store. Most important decisions happen when you don’t know what you’re going to get.
Enter Uncertainty
This is where investing lives. You don’t know if a stock will go up or down. You don’t know if the economy will crash next year. Every financial decision is a bet.
So economists came up with a way to handle this. They said: when outcomes are uncertain, people still maximize utility. They just do it in expected value terms.
Here is what that means. Say you have two choices:
Choice 1: 5% chance of winning $100, 95% chance of winning nothing.
Choice 2: 50% chance of winning $10, 50% chance of winning nothing.
Both choices pay you $5 on average. But Choice 2 is safer. You’re more likely to actually get some money.
Most people pick Choice 2. And economists say, great, that’s rational. You are risk-averse. You prefer the safer bet when the expected payout is the same.
Risk aversion is basically the idea that the pain of losing is bigger than the joy of gaining the same amount. If someone offers you a guaranteed $5 or a coin flip for $10 or nothing, most people take the guaranteed $5. Even though the coin flip has the same average payout.
This all seems fine so far. People prefer certainty. They need extra reward to take extra risk. Makes sense.
But then someone broke the whole thing.
The Allais Paradox: When Smart People Act “Irrational”
This is my favorite part of the chapter. It’s a story about two economists having lunch.
Leonard Savage was a big name in utility theory. He wrote the rules for what counts as rational behavior when making bets. Maurice Allais was his friend and colleague. Allais thought Savage’s rules were too strict. So over lunch one day, Allais set a trap.
He gave Savage two pairs of choices.
First pair:
- Lottery A: 100% chance of winning $1 million. A sure thing.
- Lottery B: 89% chance of $1 million, 10% chance of $5 million, 1% chance of nothing.
Second pair:
- Lottery A: 11% chance of $1 million, 89% chance of nothing.
- Lottery B: 10% chance of $5 million, 90% chance of nothing.
Savage picked A in the first pair and B in the second pair. And this violated his own rules.
Why? Because the second pair is just the first pair with a mathematical adjustment. You take 89% chance of winning $1 million away from both options in the first pair, and you get the second pair. If you’re truly maximizing expected utility, your preference should not change. If you prefer A in the first set, you must prefer A in the second set too. No matter what your utility function looks like.
But Savage didn’t. And most people don’t either.
In the first pair, people grab the sure million. Why gamble when you can have a guaranteed million dollars? That 1% chance of getting nothing in Lottery B feels terrifying.
But in the second pair, both options are risky anyway. You’ll probably get nothing either way. So people think, well, if I’m going to gamble, I might as well go for the big $5 million. The difference between 10% and 11% doesn’t feel that important.
This is the Allais Paradox. And it shows that people don’t actually maximize expected utility the way economics says they should.
It Gets Worse
Kahneman and Tversky ran their own version of this experiment. Different numbers, same result. People’s choices flipped when mathematically they shouldn’t have.
And then there’s an even deeper problem with the whole utility-equals-wealth idea.
Think about this. Jack and Jill both have $5 million today. Are they equally happy?
Classical economics says yes. Same wealth, same utility, same happiness.
But what if yesterday Jack had $1 million and Jill had $9 million?
Jack just went from $1 million to $5 million. He’s thrilled. His money grew five times.
Jill went from $9 million to $5 million. She lost almost half her wealth. She’s miserable.
Same wealth today. Completely different feelings about it.
This is a big deal. It means happiness isn’t just about how much you have. It’s about where you started. It’s about the change, not the level.
Classical utility theory says only the final number matters. But real humans care about the journey. Did you go up or down? By how much? From where?
Why This Matters for Your Money
You might think this is just academics arguing about math. But it directly affects how you invest.
If the rational model were true, you would never panic sell during a crash. You would calmly calculate expected values and make the optimal choice. You would never hold a losing stock just because you can’t stand to “realize” the loss.
But you do these things. We all do. Because we are not the Rational Man.
The Allais Paradox shows that even when the math is clear, even when someone explains it to you, your gut makes a different choice. You overweight certainty. You evaluate risk differently depending on how the options are framed.
And the Jack and Jill example shows that your starting point matters enormously. If your portfolio was at $100,000 last month and it’s at $80,000 today, you feel terrible. Even if $80,000 is objectively a lot of money. You don’t feel the absolute level. You feel the change.
This is the crack in the foundation of classical economics. And it’s exactly where behavioral finance stepped in.
What Comes Next
The next chapter introduces prospect theory, by Kahneman and Tversky. This is the theory that replaced expected utility as a way to describe how people actually make decisions under uncertainty.
Prospect theory says: people think in terms of gains and losses from a reference point, not in terms of total wealth. And losses hurt roughly twice as much as equivalent gains feel good.
It’s the most important idea in behavioral finance. And it builds directly on what we covered here.
The Rational Man was a useful fiction. But fiction it was.
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