Name: 株式市場は非合理的ですか？ (Is the Stock Market Irrational?)
Uploaded: 2021-01-14T11:03:56.000Z
Duration: 9 min 33 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

So GDP is down, retail sales have fallen off a cliff, and unemployment is the worst it's

ever been, but the stock market… is doing pretty good?

Have all these traders been living in a cave for the last three months?

Or maybe they're ahead of the curve, and they already know that the economy is on the

That's a nice thought, but it's probably safer to stick to Paul Krugman's three rules

about interpreting stock prices: “First, the stock market is not the economy.

Second, the stock market is not the economy.

Third, the stock market is not the economy.”

A lot of the rosy numbers are being driven by big tech companies like Amazon and Apple,

who are doing gangbusters with everyone trapped at home.

So-called “early cycle” stocks, which tend to be more reflective of the real economy,

like cars, banks and consumer goods, are lagging behind.

And thousands of small businesses crippled by the pandemic are not represented in the

Some economists describe what's happening as a market “de-coupling,” when stock

prices cease to reflect economic realities.

Most forecasters now predict a lot of bad numbers in the coming months, yet investors

The warning lights are all flashing red, but the market seems to be blissfully unaware

I thought stock traders were supposed to be really logical people,” ...well, you just

waded into a century-long debate about the very nature of the stock market.

Is it a rational mechanism that accurately reflects what companies are worth?

Or is it governed by the touchy-feely emotions and biases that all humans are subject to?

There's a long tradition of using emotional phrases to describe the world of finance.

Investors are often described as “optimistic” and “cheery,” or “skittish” and “glum.”

Alan Greenspan famously warned about traders' “irrational exuberance” during the dot

com bubble, and there's an old Wall Street saying that “financial markets are driven

by two powerful emotions: greed and fear.”

And yet, for much of the 20th century, the consensus among economists was that the market

was perfectly rational and stock prices were always an accurate reflection of a company's

According to the “efficient market hypothesis,” investors react to good or bad news about

a company, driving the price up or down until the risk is balanced with the reward.

Therefore, in theory, a company can never be “undervalued” or “overvalued.”

Since all public information about a company is already reflected in its stock price, it's

Many years earlier, John Maynard Keynes, one of the godfathers of modern economics, doubted

that a dispersed pool of investors could really know a company's true value.

He compared the stock market to an offensively outdated little contest the British newspapers

They would print the photos of a hundred women and ask readers to pick the six prettiest

The winner would be the person whose choices most closely matched the average choices of

The problem with this (aside from it being an ancestor to Hot or Not) is that shrewd

contestants would not pick the faces they actually thought were the prettiest, or even

the faces that they thought the average person would find the prettiest, but the faces that

they thought the average person would guess the average person would find the prettiest.

If you think your head is spinning now, consider another version of this dilemma called the

A bunch of contestants are asked to pick a number between 1 and 100.

The winning number is the one that is closest to 2/3rds of the average of all the numbers.

So if the average is 50, whoever picks 33 wins.

But, assuming most of the contestants can do basic math, they'll probably choose 33,

which means the winning number is now 22.

And if most of the contestants anticipate this, they'll likely choose 22, making the

And if they all guess that the others are thinking the same way, they'll choose 15

Depending on how clever the group is, the winning number can quickly dwindle down to

These examples are meant to illustrate that when people buy and sell stocks, they're

not always trying to predict something as concrete as profitability.

Many speculative traders are simply trying to guess what other people will be willing

to spend on a stock in the future, which is dependent on what those people think other

Much like the 2/3rds game, this can cause a cascading effect of over- or undervaluation.

In the 1980s, the Nobel Prize-winning economist Richard Thaler set out to prove that it was

possible for companies to be over- or undervalued by building two theoretical portfolios of

The “Winners” portfolio was made up of companies whose stock price had recently performed

exceptionally well, and the “Losers” portfolio was full of stocks that had recently performed

His theory was that these movements were fueled by investors being over-enthusiastic about

the winners and overly-pessimistic about the losers.

If he was right, then all these extreme prices would regress back to the mean, causing the

“Losers” portfolio to perform better.

In survey after survey, the Losers performed better than the Winners, often by a wide margin.

It seemed to suggest that stock prices can be influenced by what Keynes called investors'

“animal spirits,” or what psychologists might call “herd behavior”--jumping on

and off bandwagons because everyone else is.

What Thaler had tested was a form of “value investing”--betting that certain companies

The efficient market hypothesis said this should be impossible, but tell that to Warren

And then, something happened that Thaler believed settled the debate.

The day the DOW dropped more than 500 points.

On October 19th, 1987, stock prices crashed all over the world, without warning or explanation.

According to the efficient market hypothesis, stock prices are only supposed to change based

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