Security market line (SML) is the graphical representation of the Capital asset pricing model. It displays the expected rate of return of an individual security as a function of systematic, non-diversifiable risk (its beta).^[1]

Formula[link]

The Y-intercept of the SML is equal to the risk-free interest rate. The slope of the SML is equal to the market risk premium and reflects the risk return trade off at a given time:

Failed to parse (Missing texvc executable; please see math/README to configure.): \mathrm{SML} : E(R_i) = R_f + \beta_{i}[E(R_M) - R_f]\,

When used in portfolio management, the SML represents the investment's opportunity cost (investing in a combination of the market portfolio and the risk-free asset). All the correctly priced securities are plotted on the SML. The assets above the line are undervalued because for a given amount of risk (beta), they yield a higher return. The assets below the line are overvalued because for a given amount of risk, they yield a lower return.^[2]

There is a question about what the SML looks like when beta is negative. A rational investor will accept these assets even though they yield sub-risk-free returns, because they will provide "recession insurance" as part of a well-diversified portfolio. Therefore, the SML continues in a straight line whether beta is positive or negative.^[3]

Security Market Line, Treynor ratio and Alpha[link]

All of the portfolios on the SML have the same Treynor ratio as does the market portfolio, i.e.

Failed to parse (Missing texvc executable; please see math/README to configure.): \frac{E(R_i) - R_f}{\beta_i} =E(R_M) - R_f.

In fact, the slope of the SML is the Treynor ratio of the market portfolio since Failed to parse (Missing texvc executable; please see math/README to configure.): \beta_M=1 .

A stock picking rule of thumb for assets with positive beta is to buy if the Treynor ratio is above the SML and sell if it is below (see figure above). Indeed, from the efficient market hypothesis, it follows that we cannot beat the market. Therefore, all assets should have a Treynor ratio less than or equal to that of the market. In consequence, if there is an asset whose Treynor ratio is bigger than the market's then this asset gives more return for unity of systematic risk (i.e. beta), which contradicts the efficient market hypothesis.

This abnormal extra return over the market's return at a given level of risk is what is called the alpha.

See also[link]

• Capital allocation line
• Capital market line
• Market portfolio
• Modern portfolio theory
• Security characteristic line

References[link]

External links[link]

• CFA Level 1 - The Security Market Line
• RISK, DIVERSIFICATION, AND THE SECURITY MARKET LINE (SML)
• Chapter 11 Capital Asset Pricing Model (CAPM)