Is Portfolio Theory Harming Your Portfolio?

14 Pages Posted: 15 May 2011 Last revised: 10 Jun 2011

Multiple version iconThere are 2 versions of this paper

Date Written: April 29, 2011


Modern Portfolio Theory (MPT) teaches us that active equity managers who use judgment to make investment decisions won’t be able to match the returns (after fees and expenses) of blindly-invested, passively-managed index funds. Data on returns supports the theory, so it’s no surprise that investors are leaving actively managed funds in droves for the better average returns of super-diversified index strategies. Yet the reality is much murkier than we’ve been led to believe. It turns out that the portfolio theories which inspired the creation and popularity of index funds and top-down, quantitatively-driven index-like strategies, are both flawed and impractical. There’s compelling evidence, moreover, that a subset of active managers do persistently outperform indexes. However, this important fact has been lost because we allow MPT to define the debate in its own misleading terms, tilting the field in its favor and hiding the reality about active manager performance in a complex game of circular arguments.

MPT relies on a number of unrealistic assumptions including an inaccurate definition of risk. Yet this characterization of risk sets the rules for comparing active vs. passive strategies, often causing active strategies to appear more risky and less efficient than their index counterparts. The same flawed logic is used to risk-adjust returns, biasing them downward for more active, concentrated managers, and rendering this highly important measure highly suspect. Furthermore, reliance on MPT’s measure of risk pressures active managers to super-diversify. The average active fund is thus disfigured to the point where the typical "active" manager is not very active at all, casting the fund in an unfavorable light in a beauty contest versus super-efficient index funds.

Stripping away the influence of portfolio theory involves isolating and evaluating the relatively small group of equity managers who rely heavily on judgment to build concentrated equity portfolios. Empirical data from multiple studies show that these concentrated managers, in fact, persistently outperform indexes. The implications of this statement are enormous. Concentrated manager returns present the best test of whether human judgment can add value in allocating capital, and they win, convincingly. Yet while judgment has prevailed over passive investing, few have taken notice. Most investors continue to look at average active manager returns, not recognizing that these returns are minimally influenced by judgment.

Regardless of MPT’s shortcomings on both a theoretical and empirical level, its dominating influence will not easily be dislodged. MPT is deeply woven into the fabric of our financial system, its mathematical grounding and precise answers inspire confidence. Further, its application is crucial in bringing increased scale and profitability to the financial services industry. Few want to see change. As such, common sense and judgment will continue to diminish in importance as top-down, quantitative strategies and blind diversification gain investment dollars.

An informed investor should welcome this shift. As highly-diversified strategies gain assets, inefficiencies become more prevalent because share prices are increasingly driven by factors other than fundamentals. Individual investors, seeking to exploit these inefficiencies and outperform indexes, should invest in several concentrated funds with strong track records. Managers of these funds have proven themselves adept at turning inefficiencies into strong returns for their investors, and persistence data demonstrates that past performance can indicate which managers are likely to continue to outperform. Concentrated fund returns may exhibit more volatility than indexes, but we now have proof that over the long-term, good judgment will be rewarded.

Keywords: Modern Portfolio Theory, MPT, index funds, etf, passive management, active management, efficient market hypothesis, CAPM, criticism of portfolio theory

JEL Classification: G10, G14, G11, G81, G12, G24

Suggested Citation

Vincent, Scott, Is Portfolio Theory Harming Your Portfolio? (April 29, 2011). Available at SSRN: or

Scott Vincent (Contact Author)

Green River Asset Management ( email )

503 Albemarle St.
Baltimore, MD 21202
United States

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