Why should you add multiple asset classes to your portfolio? That seems like a simple question, but it’s one many investors would answer with only a vague comment about “more diversification.” It’s more precise to say you do so to increase expected returns or to decrease volatility. Sometimes these are mutually exclusive, but Harry Markowitz won a Nobel Prize for explaining that you can sometimes accomplish both at the same time. That insight is the basis for Modern Portfolio Theory.
One of the clearest illustrations of this idea can be found in Larry Swedroe’s book Think, Act, and Invest Like Warren Buffett, which I reviewed late last year. Swedroe shows how the return and risk characteristics of a 60/40 portfolio change as you slice and dice the equity allocations.
A portfolio made up of just the S&P 500 and five-year Treasuries returned 10.6% annually from 1975 through 2011, with a standard deviation of 10.8%. By gradually splitting that equity allocation into multiple asset classes (international stocks, value stocks, small caps, and commodities) the portfolio’s annual return increased 150 basis points to 12.1%,