I used to think of myself as a fairly rational investor. Then I looked back honestly at my own decision patterns — holding a losing position long past the point I'd have bought it fresh, treating a stock's old high price as if it were still its "real" value — and recognized I was running the exact scripts behavioral economists have been documenting for decades. The point of this field isn't that some people are irrational and others aren't. It's that the irrationality is systematic and predictable, which is actually useful: if the mistakes follow a pattern, you can build habits that route around them.
Loss Aversion: Losing Hurts More Than Winning Feels Good
The foundational finding here comes from Daniel Kahneman and Amos Tversky's prospect theory, the 1979 paper that eventually earned Kahneman a Nobel Memorial Prize and remains one of the most cited papers in the social sciences. The core idea: people feel the pain of a loss more intensely than the pleasure of an equivalent gain — not slightly more, dramatically more.
You can watch this play out in ordinary investing behavior. A position drops and the instinct isn't to ask whether the fundamentals actually changed — it's to avoid checking the account, or to sell just to make the discomfort stop, or to hold on stubbornly "until it breaks even," a point that conveniently keeps receding. The same bias runs in reverse with winners: people sell gains early specifically to lock in the good feeling before it can turn into a loss, even when every signal says to keep holding.
What's helped me in my own thinking is a blunt reframe: a losing trade isn't money lost so much as tuition paid for information you didn't have before. That doesn't shrink the loss in dollar terms, but it changes the question from "how do I make this feeling go away" to "what did this actually teach me, and does it change what I do next." The other habit that works is writing your rules down before you're emotionally invested in the outcome — what you'll do if a position drops 20%, what you won't do just because it's up 50% — because the version of you deciding in the moment isn't the version that should be trusted with that decision. Where you can, automate it entirely: a recurring transfer into an index fund never panics, because it was never asked to feel anything about the price.
Anchoring: The First Number Runs the Room
Anchoring is the tendency to lean on the first number you hear as a reference point, even when it's arbitrary or was chosen strategically by the other side. A house listed well above its real value still drags what feels like a "reasonable" offer upward. In a salary negotiation, whichever side names a number first tends to pull the eventual outcome toward it — which is exactly why the more experienced party usually tries to get the other side to say a number first.
The fix isn't complicated, it's just underused: do the research before you're in the room. Know what the role, the house, or the asset is actually worth from independent sources — compensation sites like Levels.fyi or Glassdoor, comparable sales for real estate — before anyone gets the chance to hand you a number that becomes your anchor by default. If you're in a position to speak first and you've done that homework, speak first. Whoever sets the anchor sets the frame the rest of the conversation happens inside.
Availability Heuristic: Recent and Memorable Isn't the Same as Likely
Your brain treats vivid, heavily-covered events as more probable than quiet, ordinary ones. A market downturn gets wall-to-wall coverage and starts to feel like the normal state of markets, even though corrections are a routine, recurring part of investing rather than an aberration. Meanwhile the risks that don't make headlines — being underinsured for disability, letting a cash-heavy position quietly lose ground to inflation — get underweighted precisely because nobody's writing alarming articles about them today.
I'd treat a headline as a signal about what's getting attention, not about what's actually likely. Before reacting to a drop, the useful question isn't how bad it feels but whether anything about the underlying business or economy has actually changed, or whether you're just responding to volume of coverage. In practice that means capping how often you check financial news, being deliberate about whose market commentary you follow, and trusting a protocol you wrote down while calm more than whatever your gut says in the middle of a selloff.
Sunk Cost: What You've Already Spent Doesn't Owe You a Future Return
The sunk cost fallacy is continuing to pour resources into something because of what you've already put in, rather than because of what it's likely to return from here. A stock down sharply gets held not because anyone still believes in the thesis, but because selling would mean "admitting" the original money is gone — as if holding it unsold somehow keeps that option alive. It doesn't. The money is spent either way; the only real question is what the position is worth doing with starting today.
The test I actually apply: would I buy this today, at today's price, with today's information, if I were starting from zero? If the honest answer is no, already owning it isn't a reason to keep it — it's a sunk cost wearing the costume of loyalty. This shows up well outside investing too — an unused subscription, a degree program that's stopped fitting, a small business that's stopped making sense — but investing is where it's most expensive to get wrong, because the "next dollar" question is always live. Every day you hold a position you wouldn't buy fresh is a day that capital isn't doing something better elsewhere.
A Few More Worth Naming
Confirmation bias is the habit of seeking out information that agrees with a position you've already taken and quietly discounting what doesn't — a real problem if your research process is mostly searching for reasons a stock you already own is a good idea. Overconfidence shows up as underdiversification and excessive trading, built on the belief that you can consistently pick winners when the broader evidence on active trading generally argues otherwise. Present bias is the pull toward spending now over saving for a future that doesn't feel real yet, which is exactly why automating a transfer before the money ever reaches your checking account beats relying on willpower after it's already there.
A Simple Check Before You Act
None of this requires memorizing the academic vocabulary. Before a financial decision with any emotional charge to it, I've found four questions do most of the work. Am I feeling something strong right now — fear, excitement, urgency — and would I still make this call in 48 hours? Which of the biases above might be shaping how I'm seeing this? What's the actual base rate here, separate from how probable it feels? And, the one that matters most: if I were starting today with no prior position, would I still choose this?
That last question strips away everything you've already sunk into a decision and asks only whether it holds up on its own. Most of the expensive financial mistakes I've watched people make — including a few of my own — don't survive that question once it's actually asked out loud.
Your biggest obstacle to building wealth usually isn't market returns. It's the set of predictable, well-documented ways your own brain misjudges risk, loss, and time. You can't switch the wiring off, but you can build habits — written rules, automation, deliberate delay — that work around it instead of against it.