The beta coefficient measures how strongly an asset’s returns tend to move with the returns on the market portfolio.
$$$$
Core formula
In the standard CAPM setup:
$$ \beta_i = \frac{\operatorname{Cov}(R_i, R_m)}{\operatorname{Var}(R_m)} $$
where (R_i) is the asset’s return and (R_m) is the market return.
This means beta is about systematic risk, not total risk. It asks how much the asset co-moves with market-wide shocks that cannot be diversified away.
Interpretation
- (\beta = 1): moves roughly with the market
- (\beta > 1): tends to amplify market moves
- (0 < \beta < 1): tends to move with the market but less strongly
- (\beta < 0): tends to move against the market
Why economists and investors care
In the capital asset pricing model, expected return depends on beta because investors are compensated for bearing market risk:
$$ E(R_i) = R_f + \beta_i \big(E(R_m) - R_f\big) $$
That makes beta central to asset pricing, portfolio design, and the cost of capital.
Related Terms
Knowledge Check
### What does beta mainly measure?
- [x] Sensitivity of an asset's return to market-wide movements
- [ ] The accounting profit of the issuer
- [ ] The maturity of a bond
- [ ] The inflation rate
> **Explanation:** Beta captures co-movement with the market portfolio, which is why it is treated as a measure of systematic risk.
### Why is beta important in CAPM?
- [x] Because expected return depends on the amount of market risk an asset carries
- [ ] Because beta replaces all other forms of analysis
- [ ] Because beta measures only idiosyncratic risk
- [ ] Because CAPM assumes all assets have beta equal to one
> **Explanation:** In CAPM, investors are compensated for systematic risk, and beta is the coefficient that measures that exposure.
### What does a beta greater than one usually imply?
- [x] The asset tends to move more than the market in the same direction
- [ ] The asset is risk free
- [ ] The asset always outperforms the market
- [ ] The asset has zero covariance with the market
> **Explanation:** A beta above one means the asset's return tends to respond more strongly than the market to common shocks.