Security Market Line (SML): Key Features and Graph Slope

Key Points:

  • The SML is a graphical representation illustrating the relationship between the expected return of securities and their risk measured by beta, based on the Capital Asset Pricing Model (CAPM).
  • The x-axis represents beta, indicating a security’s relative risk and the y-axis represents the expected return. The y-intercept is the risk-free rate, while the slope reflects the equity risk premium.
  • The SML predicts the expected return of securities relative to their risk. Securities above the SML are undervalued, while those below are overvalued.
  • The SML has several limitations, such as its single-factor risk measurement, assumption of market efficiency, and reliance on historical data.

The Security Market Line (SML) is a graphical representation used in financial markets to display the relationship between the expected return of marketable securities and their risk as measured by beta. The SML is a crucial concept in the Capital Asset Pricing Model (CAPM), which describes how market participants price securities to account for risk and the time value of money.

Components of the SML

The SML graph includes:

  • The y-axis represents the expected return of a security.
  • The x-axis shows the security’s beta, indicating its relative risk compared to the overall market.
  • The y-axis intercept represents the risk-free rate, the return of an investment with zero risk.
  • The line itself shows the required return rate for a given risk level (beta).

How Does the Security Market Line (SML) Work?

The SML serves as a predictor of a security’s return through its beta. It helps understand how much return an investor should expect for taking on additional risk. Moreover, the formula for the SML is derived from the CAPM. It illustrates the direct relationship between the beta and the expected return.

SML and Asset Pricing

The Security Market Line is fundamental in asset pricing. It provides a benchmark for evaluating whether security offers a suitable expected return for its risk level. Securities plotted above the SML are undervalued, as they yield higher returns than warranted by their risk, while those below are overvalued.

Security Market Line Graph Example (SML)

In an SML graph, the risk-free rate is where the line intersects the y-axis, and the slope extends upwards, showing increasing returns for higher risk levels. The formula to calculate the expected return using the SML is:

Expected Return=Risk-Free Rate+Beta×(Market Return−Risk-Free Rate)

This equation calculates the return by adding the product of the security’s beta and the market risk premium to the risk-free rate.

The Security Market Line (SML) graph is based on the CAPM’s assumption that there’s a linear relationship between a security’s expected return and its systematic risk, measured by beta. Therefore, the SML graph shows how the expected return of a security increases with its systematic risk.

Component of the Graph

  • The X-axis represents Beta (β), indicating systematic risk.
  • The Y-axis shows the Expected Return on a security.
  • The Y-Intercept is the Risk-Free Rate (rf), set at 3% in this example.

The slope of the line illustrates the equity risk premium (ERP), showing the excess return over the market return, which is 10% at a beta of 1.0 in this example.

Security Market Line (SML) vs. Capital Market Line (CML)

The SML applies to all marketable securities and uses beta to measure risk. At the same time, the Capital Market Line (CML) focuses on efficient portfolios, depicting risk as the standard deviation from the portfolio’s expected return. The CML represents the market portfolio and risk-free rate combinations, showing the best possible risk-return combinations.

What is the Slope of the Security Market Line?

The slope of the SML is determined by the market risk premium, which is the difference between the expected market return and the risk-free rate. The slope indicates the return required per unit of risk (beta). 

Investors and analysts use the SML to assess the performance of securities against expected market returns. Moreover, they should decide whether to buy or sell based on their positions relative to the SML.

How to Interpret the Slope of the Security Market Line?

A steeper slope on the SML suggests a higher market risk premium, indicating that investors demand more return for each unit of risk. Conversely, a flatter slope implies a lower risk premium.


CAPM is a financial model that describes the relationship between an investment’s expected return and risk. It’s used to estimate the return an investor should expect to receive on an asset based on its level of risk as measured by beta. Moreover, CAPM considers the asset’s sensitivity to non-diversifiable risk. This is also known as systematic or market risk compared to the overall market. The formula for CAPM is:

Expected Return =Risk-Free Rate+Beta×(Market Return−Risk-Free Rate)

The risk-free rate is the return on an investment that carries no risk. Beta measures the volatility of an asset relative to the market, while the market return represents the market’s expected return over the risk-free rate.

The SML graph line illustrates the expected returns of different securities based on their risk levels. The SML considers securities correctly priced, while it views those above the line as undervalued and those below it as overvalued.

Applications of the Security Market Line in Investment Decision-Making

Investors use the SML to make informed investment decisions by comparing individual securities with their expected returns based on their beta. If a security plots above the SML, it suggests the security offers a higher return relative to its risk, making it potentially attractive for investment. Conversely, investors view securities below the SML as overvalued or yielding less return for their risk level, which might indicate a need to adjust portfolios.

Furthermore, investment analysts often leverage the SML to assess portfolio risk. By understanding where different securities lie about the SML, they can make strategic decisions about asset allocation, diversification, and portfolio rebalancing.

What Are The Limitations of the Security Market Line?

The Security Market Line (SML) has several limitations despite its widespread use:

  1. Single Factor Limitation: SML only uses beta to measure risk while ignoring other influential factors like size and industry-specific risks.
  2. Market Efficiency Assumption: It assumes all markets are efficient and all securities are correctly priced, which is often untrue.
  3. Risk-Free Rate Issue: The model uses a theoretical risk-free rate, which practically does not exist.
  4. Historical Beta Issues: SML relies on historical data to predict future beta, which may not always be accurate.
  5. Homogeneous Expectations: The model assumes all investors have the same expectations, which is unrealistic.
  6. Linear Relationship Assumption: SML assumes a strict linear relationship between returns and beta, which might not always hold.
  7. Ignoring Dividend Policy: It does not consider the impact of a company’s dividend policy on stock prices.
  8. Excludes Unique Business Risks: SML overlooks specific business or financial risks unique to companies.

Investors should use SML as a starting point and incorporate other tools and analyses to assess investment opportunities thoroughly.


SML is a key tool for evaluating the risk-return relationship in securities, offering a visual representation of expected returns relative to risk levels. Investors use it to identify undervalued and overvalued securities, guiding portfolio decisions and risk assessments. Despite its simplicity, the SML has limitations, such as its reliance on beta and assumptions of market efficiency. To make informed investment choices, combining insights from the SML with other financial analysis methods is crucial, ensuring a comprehensive portfolio management approach.

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