Let’s illustrate the point with a portfolio comprised 70% of “core” equities and 30% “core” bonds (a “70/30 portfolio”). In our experience, some investors may believe that owning several different types of equity allocations can diversify their portfolios. For this reason, our hypothetical portfolio spreads the 70% of core equities among US Large Cap Value, US Large Cap Growth, US Mid Caps, US Small Caps, and International Equities.2 Core bonds are represented by the Barclays US Aggregate Bond Index, which includes a range of fixed-income investments from US Treasuries3 to investment-grade corporate bonds.
Although at first glance this portfolio may seem well diversified, it may not be as diversified as some investors may expect. As measured by the annualized volatility of returns from 2001 to 2014 (the earliest common inception of the indices examined) – that’s our definition of “risk” – over 99% of the risk of this portfolio can be explained by the 70% allocation to equities. One question at this point is why? We see two explanations. First, the bonds in this 70/30 portfolio historically have not diversified much risk in the portfolio. Second, even seemingly dissimilar parts of the equity markets such as US mid cap value and international equities are large, widely owned asset classes, which historically have exhibited high correlations in their returns.
How can investors adapt? One potential answer would be to increase their bond allocations. But the effect on risk of this approach historically has been limited. Over the same period, a decision to reverse the allocations – 30%to stocks, 70% to bonds, a “30/70 portfolio” – still left a majority of portfolio risk in equities (80%).