Why prices move on news

The day good news made a stock fall

A water bottler, Brightwater, reports its yearly results. Profit grew 6%. That’s genuinely good — more bottles sold, more money earned, a real business doing better than last year. The headlines say so. And yet, the moment the numbers come out, Brightwater’s share price drops 8% in an afternoon.

A friend who owns the shares stares at the screen. Good news. Price down. It looks broken. It isn’t. This is the single most confusing thing about markets, and once you see it, a hundred headlines stop being mysterious.

A price already knows what the crowd expects

Go back to what a price is. Item 4 taught it: a price is the latest point a buyer and a seller agreed on. It’s the crowd’s best current guess at what the slice is worth.

Here’s the part that matters. The crowd isn’t guessing blind. It already has views about the future. Everyone watching Brightwater already believed it would grow this year. Analysts said so. The price you could buy at this morning already had that expected growth baked into it.

That baking-in has a name: the news is “priced in.” It means the expectation is already inside today’s price. The crowd has already paid for the good thing it expects to happen.

So when the good thing actually happens, the price doesn’t jump. Why would it? You don’t get excited about a gift you already opened.

What moves a price is the surprise

This is the whole lesson in one line: a price moves on the gap between the news and what was already expected — not on the news itself.

Call that gap the surprise. Surprise is the actual result minus the expected result.

• Result better than expected → a positive surprise → the price rises.
• Result worse than expected → a negative surprise → the price falls.
• Result exactly as expected → no surprise → the price barely moves, even on objectively big news.

The raw number — “profit up 6%” — tells you almost nothing on its own. You can’t know if a price will rise or fall until you know what the crowd was expecting. The expectation is the reference point. The news is only good or bad relative to it.

A worked example: Brightwater’s 6%

Walk Brightwater through carefully, with numbers.

Coming into results day, analysts expected Brightwater’s profit to grow 10%. That’s not a secret. It’s the consensus — the crowd’s shared guess. And because the price reflects the crowd’s view, today’s share price already assumes 10% growth. People bought at that price expecting it.

Then the actual number lands: profit grew 6%.

Six percent is growth. The business is bigger and richer than last year. By any plain reading, that’s good news. But against the expectation of 10%, it’s a negative surprise of 4 percentage points. The crowd had paid for 10% and got 6%. They overpaid for an expectation that didn’t come true.

So sellers rush to adjust. The price that assumed 10% is now too high; it falls to a level that assumes 6%. Down 8% in an afternoon — on good news.

Now run the mirror. Suppose instead the crowd was fearful. A rumour spread that Brightwater’s spring was drying up; analysts expected profit to fall 15%. The price already reflected that gloom — people had sold, bracing for a bad year.

Then the actual number lands: profit fell only 3%.

A 3% drop is bad news. The business earned less. But against an expected fall of 15%, it’s a positive surprise of 12 points. It’s far less bad than feared. The gloom was overdone, the price was too low, and buyers pile back in. The stock rises — on a fall in profit.

Same pattern both times. The direction of the move came from the gap, not the number.

Why “buy the rumour, sell the news” makes sense

Traders have an old saying: buy the rumour, sell the news. It sounds backwards until you have the surprise idea, and then it’s obvious.

While a good thing is still only a rumour, it isn’t fully priced in yet — there’s room for the price to rise as the crowd grows confident. By the time the news is official and certain, the expectation is fully baked into the price. The surprise is gone. There’s nothing left to move on, so some who bought the rumour now sell into the confirmation. The event everyone was waiting for can pass with the price flat — or down.

You can’t beat a price by knowing the obvious

Sit with what this means. The price in front of you isn’t a fact waiting to be discovered. It’s a living summary of everything the crowd already expects. Millions of people have already poured their best guesses into it.

So knowing the obvious news — “the company is doing well,” “rates are rising,” “the product is popular” — earns you nothing. Everyone knows it. It’s already in the price. The only thing that moves a price from here is information the crowd doesn’t yet have, or has judged wrong. And by definition, that’s the hard part: you’d have to know better than the combined guess of everyone watching.

That should make anyone humbler about the headline they just read. The news that feels like a sure thing is exactly the news already spent. The market isn’t reacting to the world; it’s reacting to the difference between the world and what it already believed — and you are one small voice inside that crowd, not standing outside it. Seeing the gap, rather than the number, is the start of reading the market the way it actually moves.