This paper examines the theory and practice of investment portfolio diversification, explaining the distinction between systemic and firm-specific risk and how breadth of holdings reduces overall portfolio risk. It identifies key impediments to achieving full diversification across three dimensions: industries, asset classes, and geographic regions. Barriers discussed include limited investor knowledge, profit-taking behavior, price fluctuations, transaction costs, varying risk tolerances, and domestic bias in equity selection. The paper argues that while full diversification is often impractical, understanding its importance enables investors to make more informed decisions about balancing risk reduction against real-world constraints.
Diversification is essential to managing the risk in an investment portfolio. The underlying theory is that there are two types of risk: systemic and firm-specific. Full diversification comes from investment breadth — the underlying principle of which is that the more independent securities held in a portfolio, the better diversified it will be (Polakow & Gebbie, 2008). Different individual securities are exposed to a variety of different types of risk. The risks associated with one security can be counterbalanced by other securities that benefit under the same scenario that causes the first security to struggle. If a portfolio is sufficiently diversified, then the firm-specific risk is eliminated, leaving only the risk associated with the market as a whole. Thus, diversification reduces the total risk of the portfolio.
There are, however, impediments to creating a fully diversified portfolio. A fully diversified portfolio should contain securities from a wide variety of sectors and industries to take advantage of the differing performance of different industries under identical economic scenarios.
One impediment to achieving diversification across industries is lack of investor knowledge with regard to all industries. Some investors are more comfortable with certain industries or companies and weight their portfolios accordingly. Such portfolios may contain firms from a wide range of industries, but the weighting of the portfolio leaves it exposed to certain industries and firms with which the investor is most familiar.
Another impediment to cross-industry diversification is profit-taking. As certain industries enjoy success or failure, the temptation is to buy or sell all holdings in those industries. This can result in a portfolio that lacks balance, as holdings of certain industries become distorted by profit-taking or by excessive buying.
Price fluctuations can also impact cross-industry diversification. If an industry experiences strong price movements, it can become either an abnormally large or an unusually small component of the portfolio. These price fluctuations affect the total diversification of the portfolio. An investor may be hesitant to sell a position that is still rising, or to buy into a declining industry simply for the sake of maintaining diversification.
A related problem is that of transaction costs. Constant rebalancing may be necessary to adjust for rapid price movements in a given industry; however, constant rebalancing can be cost-prohibitive, either discouraging the investor from engaging in rebalancing transactions or negating the benefits of those transactions. It has been demonstrated that as transaction costs — calculated on a percentage basis — increase, portfolio reallocation decreases (Rowland, 1996).
"Investor needs, risk tolerance, and asset class knowledge"
"Domestic bias, information gaps, and international costs"
Knowledge and transaction costs are recurring themes — these contribute to investors' unwillingness to fully adopt portfolio diversification. Investors prefer to put their money into instruments and companies they understand. The solution is to broaden the investor's knowledge, a difficult task at times. Investors may balk at transaction costs, and therefore a rational economic decision must be made with respect to the benefits of diversification versus its costs. The impediments to diversification can be overcome once the investor understands the importance of building a fully diversified portfolio in terms of risk reduction.
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