Markowitz Portfolio Theory Its Applicable in Nigeria’s Investment Environment, Its Effectiveness Shaped by Market Inefficiencies, Limited Data, and Investor Behavior

Abstract

This article examines Markowitz Portfolio Theory as a foundational framework in modern investment analysis. The study reviews the theoretical underpinnings, empirical evidence, methodological applications, and criticisms of the theory. Using a conceptual and analytical approach, the article synthesizes prior literature to explain how diversification, risk–return trade- offs, and portfolio optimization enhance rational investment decision-making. The paper contributes to investment literature by providing an expanded, structured, and APA-compliant discussion suitable for academic and professional reference. The Markowitz Mean-Variance Model , fundamental to Modern Portfolio Theory, provides a quantitative framework for asset allocation. It meticulously outlines the principles and process for calculating the optimal portfolio solution by mapping portfolio risk (quantified by variance or standard deviation) against expected return (the mean). This academic paper critically analyses MPT, acknowledging its strengths—notably the promotion of diversification and its robust risk-return calculus—whilst addressing its significant limitations. The primary theoretical weakness stems from the model's reliance on the assumption of strong-form efficient financial markets. In reality, most markets exhibit only weak or semi-strong efficiency, being subject to informational asymmetries, insider trading, and irrational investor behaviour, which fundamentally challenge MPT's efficacy. This study specifically evaluates the relevance and practical application of Markowitz’s theory within Nigeria’s emerging investment landscape. Despite MPT’s universal conceptual relevance in finance, its core assumptions about market efficiency and investor rationality are frequently compromised in contexts such as Nigeria, where structural inefficiencies, data constraints, and unique behavioural factors prevail. Drawing upon empirical evidence and comparing MPT with alternative methodologies, such as Sh

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