How Stock Markets Work
The Market Knows Everything — Except When It Doesn't
The most influential idea in modern finance is built on an assumption that, if true, means almost no one should bother trying to beat the market — and if false, means the entire system might be flying half-blind.
The Idea
The Efficient Market Hypothesis — EMH, in the shorthand — holds that stock prices at any given moment already reflect all available information. If a company's earnings are about to surge, smart investors will have spotted the signals, bought in, and pushed the price up before the news breaks. By the time you read the headline, the market has already moved. This means that consistently outperforming the market isn't a matter of skill — it's luck, because any edge you think you have is already priced in. EMH comes in three flavours of strength. The weak form says prices already reflect all past trading data, so chart-reading is essentially astrology. The semi-strong form says prices reflect all publicly available information, so fundamental analysis — poring over annual reports — gives you no lasting advantage either. The strong form goes furthest: even private, insider information is already baked in, which most people, and most regulators, reject outright. What makes EMH genuinely interesting isn't whether it's right — it's what it implies if it's even approximately right. If markets are efficient, passive index funds should beat most active fund managers over time. And they do. Consistently. Yet billions are still paid to analysts and fund managers to pick stocks. The hypothesis has been both enormously useful and endlessly attacked — and the attackers have some very good points.
In the World
In 1973, economist Burton Malkiel published A Random Walk Down Wall Street, in which he argued that a blindfolded monkey throwing darts at a stock listing could build a portfolio that performs as well as one assembled by experts. The claim was maddening to Wall Street professionals, but the data kept vindicating it. Decades of research showed that the vast majority of actively managed funds — often around 80 to 90 percent, depending on the time horizon — underperform their benchmark index after fees. Then came the anomalies. Researchers started cataloguing moments where markets behaved in ways EMH couldn't explain. January, historically, produced higher returns than other months. Small companies consistently outperformed large ones in ways that shouldn't persist if prices were truly efficient. And then there was the 1987 crash, when global markets lost a quarter of their value in a single day — with no new information entering the system that could rationally justify such a move. Robert Shiller, who won a Nobel Prize in 2013 partly for his work challenging EMH, showed that stock prices are far more volatile than changes in actual corporate earnings warrant. Markets, it turns out, amplify human emotion — fear, euphoria, narrative — in ways that pure information theory cannot accommodate. Eugene Fama, the economist who formalised EMH and also won a Nobel that same year, essentially disagrees with Shiller. Two Nobel laureates, directly contradicting each other. The debate is very much alive.
Why It Matters
This isn't just an academic argument. It shapes how you might actually think about your own money. If markets are largely efficient, the rational response is humility: don't pay high fees for someone to pick stocks on your behalf, because the evidence suggests they can't do it reliably. Low-cost index funds — which simply track the whole market rather than trying to beat it — become the sensible default. This is the lesson that decades of data keep reinforcing. But if markets are only approximately efficient, with pockets of irrationality, then context matters. Understanding that prices embed expectations — not just facts — changes how you read financial news. A company reporting strong profits but still falling in price isn't a paradox; it means the market had already priced in even stronger profits. The news wasn't good enough. Most practically: EMH should make you deeply sceptical of anyone who promises they have a system that reliably beats the market. Sometimes people do. But mostly, in a world of millions of highly motivated, well-resourced competitors all chasing the same edges, the edge disappears almost the moment it's found.
A Question to Ponder
If markets are efficient enough that most experts can't beat them, why do we keep paying experts to try — and what does that say about the stories we need to tell ourselves to feel in control?
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