It seems intuitive that riskier investments should offer higher expected returns to compensate investors for taking on that risk. The Capital Asset Pricing Model makes a more specific and less obvious claim: not all risk deserves compensation. Only one particular kind of risk does, and the CAPM boils that kind of risk down to a single number, called beta, that's supposed to tell you exactly how much extra expected return a given stock deserves.
Diversifiable risk doesn't get paid for
The CAPM's central insight builds directly on portfolio theory: an individual stock's total risk includes both risk specific to that company (a factory fire, a product recall, a lawsuit) and risk that affects the broader market as a whole (a recession, a sharp interest-rate change). Company-specific risk, the CAPM argues, can be largely eliminated simply by holding a diversified portfolio of many stocks — if one company's factory burns down, that loss is offset, on average, by unrelated good news at other companies in the portfolio. Because a rational investor can eliminate this "diversifiable" risk for free just by holding a broad enough portfolio, the CAPM concludes the market shouldn't reward investors with extra expected return for bearing it — nobody has to bear it in the first place, so there's no reason the market would compensate for it.
Beta measures the risk you can't diversify away
What's left after diversification is market risk, or systematic risk — the risk tied to broad economic and market movements that no amount of diversification within stocks can eliminate, since a genuine market-wide downturn affects essentially every stock at once. The CAPM measures a specific stock's exposure to this systematic risk using beta: a number describing how sensitively that stock's returns tend to move relative to the overall market's returns. A beta of 1 means a stock tends to move in line with the market; a beta above 1 means it tends to amplify market swings, and a beta below 1 means it tends to dampen them. According to the CAPM, a stock's expected return should be directly proportional to its beta — higher systematic risk exposure, and only that kind of risk, is what justifies a higher expected return, giving investors a specific formula for what return a given level of market risk exposure "should" command.
What we're still unsure about
The CAPM's underlying logic about diversifiable versus systematic risk is well understood and forms a foundational part of modern finance theory and teaching. What remains genuinely and persistently contested is how well the model's specific empirical predictions actually hold up against real market data — decades of empirical testing have found that beta alone often explains stock returns less completely than the theory predicts, and other factors (company size, valuation ratios, momentum) have repeatedly been found to add explanatory power beyond beta, which has led to an ongoing, unresolved debate in financial economics over whether the CAPM should be treated as a useful simplification, a genuinely incomplete model, or something that needs a fundamentally different successor.
This sits inside Risk, Return & the CAPM, one of eight topics in Finance, one of five domains in Economics, one of seventeen subjects the app can quiz you on.