Glossary

Modern Portfolio Theory (MPT)

Modern Portfolio Theory is a framework introduced by Markowitz (1952) for constructing portfolios that maximise expected return for a given level of risk, or minimise risk for a given expected return. MPT formalises diversification by showing that portfolio risk depends not on...

Definition

Modern Portfolio Theory is a framework introduced by Markowitz (1952) for constructing portfolios that maximise expected return for a given level of risk, or minimise risk for a given expected return. MPT formalises diversification by showing that portfolio risk depends not only on individual asset variances but critically on the covariances between assets. The set of optimal portfolios traces an efficient frontier. MPT laid the foundation for much of modern financial economics, including the Capital Asset Pricing Model (CAPM).

Why It Matters

Before MPT, investment decisions were typically based on selecting individual securities with attractive risk-return profiles. Markowitz demonstrated that the risk of a portfolio is not simply the average risk of its constituents — it is the weighted combination of variances and covariances. This insight makes diversification mathematically precise: holding assets with low or negative correlations reduces portfolio risk without sacrificing expected return. MPT remains the conceptual backbone of asset allocation, risk management, and regulatory capital frameworks, even though its assumptions (normally distributed returns, static correlations) are often violated in practice.

Example

An investor considers two assets: a stock with expected return 10% and volatility 20%, and a bond with expected return 4% and volatility 8%. If their correlation is 0.2, a portfolio of 60% stock and 40% bond has an expected return of 7.6% but a volatility of only 13.4% — lower than the weighted average of individual volatilities (15.2%). The diversification benefit arises because the two assets do not move in lockstep.

Related Terms

Software Notes

  • SPSS: Not a primary portfolio analysis tool; export returns data to R or Python for portfolio optimisation
  • R: portfolio.optim() from the tseries package; efficient.frontier() from the PortfolioAnalytics package for comprehensive MPT analysis
  • Stata: No native portfolio optimisation commands; use mata for matrix operations or export to R/Python