"[It] grew out of the University of Chicago’s finance department, and long held sway in academic circles, that the stock market can’t be beaten on any consistent basis because all available information is already built into stock prices. The stock market, in other words, is rational.
In the last decade, the efficient market hypothesis, which had been near dogma since the early 1970s, has taken some serious body blows. First came the rise of the behavioral economists, like Richard H. Thaler at the University of Chicago and Robert J. Shiller at Yale, who convincingly showed that mass psychology, herd behavior and the like can have an enormous effect on stock prices — meaning that perhaps the market isn’t quite so efficient after all. Then came a bit more tangible proof: the dot-com bubble, quickly followed by the housing bubble. Quod erat demonstrandum."
The story is mainly centers on Jeremy Grantham of GMO "an institutional asset management company".
"As Mr. Grantham sees it, if professional investors had been willing to acknowledge these aberrations — and trade on the fact that the market was out of whack — they should have been able to beat the market. But thanks to the efficient market hypothesis, no one was willing to call a bubble a bubble — because, after all, stock prices were rational. “It helped mold the ‘this time it’s different’ mentality,” "
That said, the market is still very difficult to consistently beat (maybe because we are all biased) and thus as Grantham also states that passive index investing is still often the best way to invest due to lower costs and to protect ourselves from ourselves.