In 1990 Harry Markowitz, Merton Miller, and William Sharpe shared the first Nobel Prize in the very young area of financial economics. The Nobel committee recognized Harry Markowitz for developing portofolio theory, Miller for the theory of corporate finance, and Sharpe for the Capital Asset (stock market) Pricing Model also known as CAPM.

CAPM was the crowning acheivment of theoretical economists bent on proving that markets are efficient and work together mathematically with the precision and elegance of a Rolex watch. In the 1980s, researching financial economists began to notice a slew of empirical results that are not consistent with the view that stock market returns were determined in accordance with CAPM and stock market efficiency.

It is useful for you to understand what CAPM is because you will read or hear about it as you progress as a stock market investor. CAPM is a regression model designed to separate out the general stock market price changes from price changes specific to a given stock. The general stock market price change is called unsystematic risk. An investor can get the same return as the general stock market buying a mutual fund that is indexed to the stock market such as the Vanguard 500 fund (symbol VFINX). For this reason the amount of profit you receive on a specific stock that is as much as the stock market indexes is said to not be priced into the stock in terms of the risk you are taking.

The amount you make or lose on a given stock as compared to the stock market averages is considered to be priced by investors to compensate for the additional risk you take in buying stock in a single company instead of a fund indexed to the stock market. The profit or loss that you receive as compared to the stock market is called systematic risk. The capital asset pricing model measures systematic risk with a regression coefficient called beta. When I talk about beta now you know what it is; it is nothing more than a measure of additional potential return an investor should receive for purchasing a single stock based on how risky that stock is. I want to emphasize that CAPM is based on the notion that the stock market efficiently translates all information known about the stock market into stock prices for stock investing purposes.