As an undergraduate Fama worked for a stock forecasting service and he was tasked with coming up with rules to make money in the market. Time and time again he would find profitable rules only to find that they didn’t work in new data or out of sample. In graduate school he started talking to Merton Miller, Lester Telser, and Benoît Mandelbrot and finally hit on the idea that in an efficient market price changes would not be forecastable. The rest is history.
Fama’s dissertation and famous 1970 review article, Efficient Capital Markets: A Review of Theory and Empirical Work made efficient markets a touchstone for modern economists and finance theorists but practitioners hated and still hate the idea. Nevertheless, test after test showed that very few mutual fund managers beat the market and those that beat the market this year are not more likely to beat the market the next year. Chance and perhaps a few, very rare, geniuses explain the data. Eventually, hundreds of billions of dollars began to flow into index funds and today index funds manage over $7 trillion dollars worth of assets worldwide, making Fama the 7 trillion dollar man. Fama’s ideas have made an enormous contribution to how people invest, saving them billions in fees which generated beautiful homes for fortunate mutual fund managers but less than nothing for their customers.
The no free lunch principle is the most robust of the findings of the early Fama/efficient markets school. Other early findings such as non-forecastability of returns have been revised. The initial finding was that returns were not forecastable and that is true for short durations but it is now clear that returns can be forecastable over longer horizons! In particular, variables such as the dividend/price ratio can predict stock return variation years in advance! (Robert Shiller pioneered many of these kinds of studies as did Campbell and Cochrane). Fama, however, contrary to how he is sometimes represented did not reject these findings. Indeed, the less well known part of the story is that Fama working with French (e.g. Fama and French (1988a,1988b, 1993) has been among the pioneers in documenting and explaining these findings. What Fama’s later work has shown is that many of the anomalies such as time varying returns and the higher return to so-called value firms are real but they are not anomalies they are better explained as variations in risk premia tied to changes in the business cycle.
The CAPM (for which Markowtiz and Sharpe won the Nobel) suggested that the only source of true (priced) risk was risk that varied with the market return. That is one important source of risk but it’s not the only one, other types of macro risk which appear to vary with the business cycle are also priced and they are correlated with markers like the dividend/price ratio and the prospects for value and small-cap stocks. Thus, Fama showed that many of the seemingly anomalies (not all, however!) of the early efficient market tests can be better explained by a market model that incorporates more sources of risk. All of this work has really been a tour de force. It’s not often that the same person creates the theory and then participates in the first revolution overturning (some of) that theory.
Fama also pioneered the first event study! Fama, Fisher, Jensen, and Roll (1969) studied something a bit prosaic, stock splits, but the methodology, looking at how the stock market reacts to unexpected events, has seen been used to study what happens when Senators die unexpectedly (firms they support fall), what happens in close elections, which part was responsible for the Challenger space shuttle crash and many other events.