efficient market hypothesis
/ E-M-H /
Suppose you read in the news that a company just discovered a blockbuster drug. Can you make easy money by rushing to buy its shares? The efficient market hypothesis says no — by the time you act, the price has already jumped to reflect the good news, because thousands of professionals saw it the moment it broke. The idea, in one line, is that asset prices already absorb all available information, so you cannot reliably beat the market using what everyone can already know.
The logic is a kind of self-policing competition. With millions of investors all hunting for bargains, any piece of useful information gets traded on within seconds and baked into the price. If a stock were obviously underpriced, buyers would pour in and lift the price until the bargain vanished. The upshot is that the current price is the market's best collective guess of true value, and future price moves depend on tomorrow's unpredictable news — making prices behave like a random walk, impossible to forecast from past patterns. Economists distinguish degrees: the weak form says past prices hold no clues; the semi-strong form says all public information is already priced in; the strong form says even private information is too (almost no one believes that last one).
If the hypothesis is broadly right, the practical advice is humbling: most people, including most professionals, cannot consistently pick winning stocks after fees, so the smart move is to buy a cheap, diversified index fund and stop trying to be clever. That logic powered the rise of passive investing. But the EMH is hotly contested. Markets clearly overshoot into bubbles and crashes, behavioral economists show investors are far from coldly rational, and a few investors have beaten the market for decades. The honest verdict: markets are remarkably hard to beat and process information fast, but 'perfectly efficient' is an idealization that the real world only roughly approximates.
A company announces surprise record profits at 9:00 a.m. By 9:00:02, before any small investor can phone a broker, the share price has already leapt to reflect the news. The information was good, but the chance to profit from it vanished in seconds — exactly what the efficient market hypothesis predicts.
Prices adjust to news almost instantly, leaving little easy profit on the table.
Efficient does not mean prices are correct or rational — only that they reflect available information. Markets can be efficient in that narrow sense and still inflate into bubbles, because the 'information' the crowd is pricing in can itself be mistaken.