The Papers that Changed Investing: Anticipated Prices and Random Fluctuation

It's 1965. Computers fill entire rooms. On Wall Street, armies of analysts pore over balance sheets, earnings reports, and economic data — hunting for the next big winner.

The belief is universal: with enough research and skill, you can predict where stock prices are heading.

But here's the problem: if markets are full of intelligent people all competing to find the same opportunities, why do prices move so unpredictably?

One MIT economist asked that question — and his answer would shake the foundations of Wall Street.

His name was Paul Samuelson. Welcome to The Papers That Changed Investing.

By the mid-1960s, researchers had noticed something troubling. Stock prices showed no reliable patterns. They seemed to wander aimlessly — like a drunk staggering home.

The French mathematician Louis Bachelier spotted this randomness back in 1900.

At the same time as Samuelson, Eugene Fama was gathering fresh statistical evidence. But observation isn't explanation.

Samuelson set out to prove the answer — not with statistics, but with pure mathematics.

Samuelson's insight was elegant — and counterintuitive. Randomness isn't a flaw in the market. It's evidence the market is working.

Here's the logic. If everyone knows a stock will rise tomorrow, what happens?

Traders don't wait. They buy today. That buying pressure pushes the price up immediately. The predicted rise vanishes before it arrives.

Any pattern that can be spotted gets traded away the instant someone spots it. Competition destroys predictability.

Samuelson called this "properly anticipated" prices. When prices fully reflect all available information, only one thing can move them: news.

And news — by definition — is what nobody expected. If you could have predicted it, it wouldn't be news.

Think of it like this. A twenty-dollar bill falls on a busy high street. How long does it stay there?

Someone picks it up instantly. You never see money lying around in crowded places — the opportunity vanishes the moment it appears. Markets work the same way.

So what did Samuelson demonstrate?

First: when markets work properly, the best forecast of tomorrow's price is today's price.

Second: this doesn't mean prices are always correct, but it means they are fair, In that any predictable movement has already been acted upon.

Third: trying to forecast price changes isn't just difficult — it's mathematically unfounded. The randomness is proof that competition is doing its job.

Here's why this matters for your money today.

Samuelson showed that chasing market predictions isn't just difficult — it's a game where consistent success is not supported mathematically.

Samuelson put it memorably: "Investing should be like watching paint dry or grass grow. If you want excitement, take $800 and go to Las Vegas."

In 1970, Samuelson became the first American to win the Nobel Prize in Economics.

His proof gave ordinary investors a powerful new perspective: that broadly diversified,  low-cost  index funds may offer a way to participate in overall market returns without relying on predictions.

This research is widely cited as the intellectual foundation of index investing.

Curious how Samuelson's insight relates to modern investing concepts? The answers may be simpler than you think.

Connect with an Advisor now!

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Source

Samuelson, P. A. (1965). Proof That Properly Anticipated Prices Fluctuate Randomly. Industrial Management Review, 6(2), 41–49.

Visit ifa.com/academic-papers

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