Modern portfolio theory meaning

In legal practice on investments, modern portfolio theory (MPT) describes the finance framework commonly relied on to justify diversification and to set risk–return objectives in mandates for trustees, pension scheme managers, charities and other fiduciaries. It evaluates assets by expected return, expected risk (usually measured by standard deviation), and correlations, to construct “efficient” portfolios delivering the highest expected return for a chosen level of risk. MPT is not defined in legislation or case law; it is a descriptive theory referenced in investment policies, statements of investment principles, charity investment strategies, investment management agreements and expert evidence to demonstrate prudence and suitability. Across England & Wales, Scotland, Northern Ireland and Ireland, usage is broadly consistent. The approach aligns with fiduciary duties to act prudently, consider suitability and diversify (for example, under the Trustee Act 2000 and UK pensions investment regulations, and the Irish prudent person/IORP II regime). Key features and significance: - models variance and correlations to evidence diversification benefits; - focuses on risk‑adjusted return (for example, the Sharpe ratio) and the efficient frontier; - informs strategic asset allocation, risk limits, benchmarking and due diligence on whether a proposed portfolio is efficient for the agreed risk budget.