An MIT-trained Harvard psychologist ran an experiment that exposes a bug in how you think about money. Drive across town to save $100 on a $200 stereo? Most people say yes. Drive across town to save the exact same $100 on a $31,000 car? Most people say no.

Same $100. Same drive. Same person. The only thing that changed is what the $100 is standing next to.

His name is Dan Gilbert, Harvard psychologist, author of Stumbling on Happiness. He didn’t build this from theory — he built it on a rule from 1738. Daniel Bernoulli’s original idea for making any decision under uncertainty was simple: multiply the odds of an outcome by how much it’s worth to you.

Expected value = probability × value.

Gilbert’s argument is that we’re bad at both halves of that equation. We misjudge the odds. And we misjudge the value even worse.

The car stereo isn’t a one-off trick. It’s mental accounting — your brain doesn’t evaluate $100 as $100. It evaluates $100 as a percentage of whatever it’s attached to. Save 50% on a stereo feels like a win. Save 0.3% on a car feels like nothing. The dollar amount never moved.

Same failure shows up with a lost ticket. Lose a $20 ticket on the way to a show, and most people won’t buy a replacement. Lose a $20 bill on the way to the same show, and most people buy the ticket anyway. Economically, you’re $20 poorer either way. Psychologically, replacing the ticket feels like paying twice for one seat.

It shows up again with a wine list. Put a $27 bottle between an $8 bottle and a $33 bottle, and it looks reasonable. Change what’s sitting next to it, and the same $27 bottle can look expensive or cheap without a single cent changing.

Here’s the part that should worry you more than any of the individual examples. Every one of these comparisons — the stereo, the wine, the wallet, the ticket — disappears the moment you actually use what you bought. You don’t drink the $8 and $33 bottles next to the $27 one. You just drink the wine. The comparison that decided your purchase evaporates the second the purchase is real.

Your brain isn’t running the math wrong by accident. It’s running a completely different equation than the one it thinks it’s running — and the input it’s optimizing for vanishes right after you pay.