Capital Asset Prices: A Theory of Market Equilibrium under Conditions of Risk

Source: Sharpe, W. F. (1964). The Journal of Finance 19(3), 425–442. · DOI 10.1111/j.1540-6261.1964.tb02865.x

TL;DR

Derives the Capital Asset Pricing Model (CAPM): in equilibrium, an asset's expected return is the

pure interest rate plus its systematic risk (its responsiveness to the market) times the price of

risk. Only systematic, non-diversifiable risk is priced; the uncorrelated ("unsystematic") component

earns nothing. The foundation of modern asset pricing and of the idea of risk-adjusted return.

At the time there was "no theory describing the manner in which the price of risk results from the

basic influences of investor preferences" — traditional finance simply asserted a market risk

premium. Sharpe asks: extending Markowitz/Tobin mean-variance choice to market equilibrium, what

component of a single asset's risk actually determines its price?

Investors are single-period mean-variance maximizers facing a common pure interest rate P and able to

borrow/lend freely (homogeneous expectations, frictionless market). All hold the same efficient

combination of risky assets plus the riskless asset, so portfolios lie on the capital market line

C_R = S(E_R − P): expected return is linear in total risk for efficient portfolios. For an individual

asset i inside efficient combination g, tangency forces a linear relation between E_Rᵢ and B_ig, where

B_ig is the asset's responsiveness to the combination's return. Sharpe splits each asset's risk into a

component correlated with the combination (systematic risk) and an uncorrelated remainder

(unsystematic risk), and shows only the former matters for price. In modern notation:

E[Rᵢ] = R_f + βᵢ(E[R_m] − R_f), βᵢ = Cov(Rᵢ, R_m)/Var(R_m).

Two-fund separation: every investor holds the riskless asset plus one common risky combination.

Expected return on an asset is linear in its systematic risk (beta), not its total variance.

Idiosyncratic/unsystematic risk is diversified away and commands no premium.

The price of risk (slope of the capital market line) is common to all assets.

Provides the vocabulary of the field: beta, the market risk premium, and alpha (return beyond CAPM).

The benchmark every later factor model (Fama–French and beyond) is measured against; underlies index

investing, cost of equity, and performance evaluation.

The empirical security market line is too flat — low-beta stocks earn more, high-beta less, than

CAPM predicts (basis for betting-against-beta). Size, value, and momentum are CAPM "alphas" it cannot

explain.

Strong assumptions: single period, homogeneous beliefs, frictionless markets, common borrowing/lending

rate, mean-variance utility.

Single-factor: only one source of priced risk; no intertemporal hedging demands (cf. Merton ICAPM).

The true market portfolio is unobservable (Roll critique).

Markowitz, H. (1952/1959) — Portfolio Selection — Journal of Finance / Wiley

Tobin, J. (1958) — Liquidity Preference as Behavior Towards Risk — Review of Economic Studies

Lintner, J. (1965) — The Valuation of Risk Assets... — Review of Economics and Statistics

Fama, E. & French, K. (1992) — The Cross-Section of Expected Stock Returns — Journal of Finance

Provenance: verified/generated from the paper's full text.