From the perspective of holding a well-diversified investment portfolio according to scientific investment principles, the objective of diversification is to minimize or eliminate ‘unsystematic risk’ or those risks that are not related to the price volatility of the overall securities markets.
When people speak of investment diversification, they may mean different things. Therefore, clear definitions are important. Unsystematic risk is the risk that relates to company-specific risk factors, such as new or increased competition, labor strikes, faulty management decisions, adverse technological changes, etc.
By holding a very broadly diversified portfolio containing the securities of numerous companies in different economic spheres, the risk in your portfolio can be dramatically reduced. By holding the full market in your portfolio through broad-based index funds, unsystematic risk can be fully eliminated.
Unsystematic risk can be eliminated, because there is not a one-to-one correlation between the opportunities and risk factors that affect each particular firm. To the extent that you hold more than one firm in your portfolio and particularly a very large number of them, then company specific price movements tend to cancel out the securities price fluctuations of other firms. When fully diversified, securities market risk measured by its market price volatility will remain.
When you hold the entire market as your investment portfolio, then you achieve a very significant reduction in the price volatility of your overall personal investment portfolio.
What remains then is only the ‘systematic risk’, or the impact of broader economic, policy, and political risk factors, such as general changes in economic growth, monetary policy, inflation, taxation, wars, exchange rate fluctuations, etc. A well-diversified portfolio is still subject to these systematic risks.
In summary, you diversify to eliminate the company specific risks to your investment portfolio. You also do this, because the market does not compensate you for company specific risk. Equity risk premiums are paid to investors, because they are willing to expose themselves to market risks and not to company specific risk.
Sometimes investors believe that diversification means holding a hodge-podge of mutual funds or exchange-traded funds ( ETF ) with different styles such as growth, value, small cap, balanced, international, emerging markets, etc.
While holding multiple mutual funds of differing styles can contribute significantly to diversification in the investment science sense, the real question is whether this is the most efficient approach after investment costs and taxation are taken into account. Instead, the overall market portfolio [...]