Security Market Line: CAPM Expected Return, Beta, and Relative Valuation

What is the Security Market Line?

The Security Market Line, or SML, is the graphical representation of the Capital Asset Pricing Model.

It relates an asset's expected return to its beta:

1E(Ri) = Rf + βi[E(Rm) - Rf]

On the graph:

1horizontal axis -> beta
2vertical axis   -> expected return

The intercept is the risk-free rate, and the slope is the Market Risk Premium.

The SML therefore shows how much expected return CAPM assigns to different levels of systematic market risk.

What the intercept and slope mean

At beta zero:

1E(Ri) = Rf

At beta one:

1E(Ri) = E(Rm)

because a beta-one asset has the same modeled systematic exposure as the market portfolio.

If the risk-free rate is 4% and the expected market return is 10%, then the SML slope is:

110% - 4% = 6%

A beta of 1.5 maps to:

14% + 1.5 × 6% = 13%

A beta of 0.5 maps to:

14% + 0.5 × 6% = 7%

These are CAPM expected returns under assumed inputs, not guaranteed realized returns.

The SML uses beta, not total volatility

The SML is built on Systematic Risk.

Beta measures how sensitive an asset is to movements in the selected market benchmark.

The SML does not reward an asset merely for having high total volatility.

A stock can have high company-specific volatility and still have a moderate beta if much of its risk is Nonsystematic Risk.

Under CAPM, investors can diversify company-specific risk away, so it should not command a separate expected return premium.

This is one of the model's central economic claims.

Above or below the SML

Analysts often compare an asset's own expected return estimate with the CAPM return implied by its beta.

If the analyst estimates:

1Expected return > CAPM required return

the security may be described as plotting above the SML.

If:

1Expected return < CAPM required return

it may be described as plotting below the SML.

Textbooks often call a security above the line undervalued and a security below the line overvalued because prices should adjust until expected returns align with systematic risk.

That interpretation depends on the expected-return estimate, beta, risk-free rate, market risk premium, and CAPM itself being appropriate.

A plotted deviation is not automatically an executable alpha signal.

SML deviations and alpha

The vertical difference between an asset's return and its CAPM benchmark is closely related to Alpha.

A simplified CAPM alpha can be written as:

1αi = Ri - [Rf + βi(Rm - Rf)]

For historical data, this compares realized return with a beta-adjusted benchmark.

For forward-looking analysis, it can compare an analyst's expected return with the CAPM required return.

A positive estimated alpha does not prove the security is mispriced.

The model may omit relevant systematic factors, beta may be misestimated, the market proxy may be wrong, or the expected-return forecast may simply be inaccurate.

The SML applies to individual securities and portfolios

Unlike the Capital Market Line, the SML can be applied to individual securities as well as portfolios under CAPM.

That is because beta measures systematic risk, which can be defined for any asset relative to the market benchmark.

The CML, by contrast, describes efficient combinations of the risk-free asset and market portfolio in expected-return versus total-volatility space.

The distinction is:

1SML -> expected return versus beta; all assets and portfolios under CAPM
2CML -> expected return versus total volatility; efficient market/risk-free combinations

Confusing these two lines is one of the most common portfolio-theory mistakes.

A security can have high volatility but a modest beta

Consider two stocks.

Stock A is a mature cyclical company whose return moves closely with the market.

Stock B is a small biotechnology company whose price is dominated by company-specific trial outcomes.

Stock B might have much higher total volatility but a lower or noisier beta.

Under CAPM, only the portion of risk captured by beta affects required expected return.

This illustrates why the SML does not plot standard deviation on the horizontal axis.

The market risk premium rotates the line

The SML slope equals:

1E(Rm) - Rf

If the assumed market risk premium rises, the line becomes steeper.

Higher-beta assets receive a larger increase in required expected return than lower-beta assets.

For example, if the market risk premium rises from 5% to 7%, the required return of a beta-1.5 asset rises by:

11.5 × 2% = 3 percentage points

while a beta-0.5 asset rises by only:

10.5 × 2% = 1 percentage point

That comparative statics exercise is useful, but the market risk premium itself must be estimated.

The risk-free rate shifts the line

Holding the market risk premium fixed, a higher risk-free rate shifts the SML upward.

Every beta level gets a higher required expected return.

In practice, however, changing rates can also affect the market risk premium, company cash flows, valuations, and observed betas.

The clean parallel shift is therefore a model exercise, not a complete market forecast.

Beta estimation changes where a security plots

A security's SML position depends heavily on beta.

Beta estimates can change with:

• market benchmark;
• sample length;
• return frequency;
• treatment of outliers;
• changes in financial leverage;
• changes in business mix; and
• market regime.

An analyst using two years of weekly returns may obtain a different beta from another analyst using five years of monthly returns.

Neither estimate should be mistaken for a permanent property of the company.

The market benchmark is part of the model

CAPM refers to the market portfolio, but the theoretical market portfolio is not directly observable.

Analysts therefore use proxies.

A stock's beta relative to the S&P 500 can differ from its beta relative to a global equity index.

The chosen proxy also affects the market return and market risk premium.

Changing the benchmark can move both the horizontal position and slope used in SML analysis.

SML versus a regression line

The SML is an equilibrium expected-return relation.

It is not simply the same thing as a historical regression line through security returns.

A historical beta is usually estimated from a regression of an asset's excess returns on market excess returns.

That regression helps estimate the beta input.

The SML then uses that beta together with an assumed risk-free rate and expected market risk premium to determine the CAPM expected return.

Historical data and equilibrium expected-return logic are related but distinct steps.

SML and portfolio construction

The SML can serve as a benchmark for asking whether a security's expected return appears adequate for its systematic risk.

But portfolio construction still requires more than one point estimate.

Investors may care about:

1correlations
2liquidity
3factor exposures
4taxes
5position limits
6tracking error
7drawdowns
8cash-flow needs

A security that looks attractive relative to the SML can still worsen portfolio concentration or introduce unwanted exposures.

CAPM is not the only pricing model

CFA Institute treats CAPM as an important starting point rather than the only viable asset-pricing model.

Factor Model approaches can include several systematic dimensions beyond broad-market beta.

If value, size, momentum, rates, inflation, or other factors explain returns, a security can appear to have CAPM alpha while actually carrying compensated exposure to omitted factors.

The SML is therefore one benchmark among several possible asset-pricing frameworks.

A worked comparison

Suppose:

1Rf = 3%
2Market risk premium = 6%

Stock A has beta 0.8 and an analyst expected return of 9%.

CAPM required return is:

13% + 0.8 × 6% = 7.8%

The analyst estimate is 1.2 percentage points above the line.

Stock B has beta 1.4 and analyst expected return of 10%.

CAPM required return is:

13% + 1.4 × 6% = 11.4%

The analyst estimate is 1.4 percentage points below the line.

Under those inputs, Stock A looks more attractive relative to CAPM.

That conclusion can reverse if the expected returns or betas are wrong.

What the Security Market Line cannot tell you

The SML does not reveal a security's true future return.

It does not prove a stock above the line is undervalued.

It does not guarantee CAPM alpha will be realized.

It does not make beta stable.

It does not make the market risk premium directly observable.

It does not capture every systematic risk factor.

It does not account automatically for transaction costs, taxes, liquidity, or portfolio constraints.

The SML is best used as a disciplined benchmark for understanding the CAPM relationship among beta, required expected return, and the market risk premium.

Grizzly Bulls' Models can provide broader systematic-research context, while the Macroeconomic Conditions Index can frame market regimes. Neither route publishes a canonical live SML, stock-specific CAPM alpha, or guaranteed mispricing signal.

Sources and further reading

• CFA Institute: Portfolio Risk and Return: Part II, 2026 curriculum
• CFA Institute: Using Multifactor Models, 2026 curriculum
• CFA Institute Research & Policy Center: CAPM 60th anniversary discussion

Explore more topics in the Financial Research Encyclopedia.