Security market line
The Security Market Line (SML) is a graphical representation of the Capital Asset Pricing Model (CAPM), depicting the linear relationship between the expected return of a security or portfolio and its systematic risk, as measured by beta.[1][2] Developed in the 1960s by William Sharpe, John Lintner, and Jan Mossin in the context of modern portfolio theory, the SML originates from the equilibrium conditions where investors hold diversified portfolios and price assets based on non-diversifiable risk.[1][3] It serves as a benchmark for evaluating whether securities offer appropriate compensation for their risk exposure in efficient markets. The SML is defined by the CAPM equation:E(R_i) = R_f + \beta_i [E(R_m) - R_f]
where E(R_i) represents the expected return on security i, R_f is the risk-free rate, \beta_i is the security's beta (the covariance of its returns with the market divided by the market's variance), and [E(R_m) - R_f] is the market risk premium.[2] In this framework, the line's intercept is the risk-free rate, and its slope reflects the additional return demanded for bearing market risk.[4] Securities plotting above the SML are considered undervalued (offering higher returns for their beta), while those below are overvalued, guiding investment decisions toward mispriced assets.[4] The model assumes beta captures all relevant systematic risk, ignoring idiosyncratic risk that can be diversified away.[2] In practice, the SML finds applications in corporate finance and investment management, such as estimating a firm's weighted average cost of capital (WACC) for capital budgeting, valuing equity securities, and assessing portfolio performance relative to benchmarks.[5] For instance, analysts use it to determine required returns on projects or stocks by inputting estimated betas and prevailing market premiums.[5] However, the SML's validity depends on CAPM assumptions, including rational and risk-averse investors, homogeneous expectations about asset returns, perfect capital markets without taxes or transaction costs, and the ability to borrow and lend at the risk-free rate.[6] Empirical tests have shown deviations, such as a flatter SML than predicted, prompting extensions like multifactor models.