What To Do When Your Stocks And Bonds Portfolio Reaches Minimum Volatility
Summary Investors typically increase exposure to bonds as they near retirement, hoping to reduce volatility and drawdown risk. It is very possible to reach a point where further increasing exposure to bonds will increase rather than decrease volatility. This phenomenon is more likely to occur with longer duration bond funds. Once you reach minimum volatility for a two-fund stocks and bonds portfolio, you can further reduce risk by (1) buying treasuries or (2) switching to a shorter term bond fund. There is no general result for which strategy is preferred, but (2) tends to give better returns and may be easier to implement. Expected Returns and Volatility as you Increase Bond Exposure Suppose you are implementing a basic stocks and bonds portfolio comprised of two Vanguard mutual funds: Vanguard 500 Index Fund Investor Shares (MUTF: VFINX ) and Vanguard Long-Term Bond Index Fund (MUTF: VBLTX ). Using historical data going back to Feb. 28, 1994, here is how expected returns and volatility of the VFINX/VBLTX portfolio vary with asset allocation. (click to enlarge) Here the top-right point represents 100% VFINX/0% VBLTX; the next data point is 90% VFINX/10% VBLTX; and so on until the bottom-most point, which is 0% VFINX/100% VBLTX. As you near retirement, you may increase your VBLTX allocation to reduce risk. If you go from 90% VFINX/10% VBLTX to 60% VFINX/40% VBLTX, for example, you reduce your expected returns a little (0.041% to 0.037%), while reducing volatility considerably (1.06% to 0.70%). Further increasing the VBLTX allocation reduces volatility, but only to a point. At 25.8% VFINX/74.2% VBLTX, you reach the leftmost point on the curve, and further increasing VBLTX allocation actually increases volatility while reducing expected returns. Of course, there is never a good reason to increase volatility and decrease expected returns. So looking back at the past 21.5 years, you would never have wanted to allocate more than 74.2% to VBLTX in a VFINX/VBLTX portfolio. Longer Duration Bond Funds Have Lower Critical Points The expected returns vs. volatility curve doesn’t always have a clear critical point like we saw for VFINX/VBLTX. In general, longer duration bond funds are more likely to exhibit this phenomenon. You can see this when you compare the curve for VFINX paired with VBLTX to VFINX paired with Vanguard’s short-term and intermediate-term bond funds, VBISX and VBIIX . (click to enlarge) Looking at the blue curve, VFINX/VBISX does have a minimum volatility point, but it’s at a very high VBISX allocation (4.3% VFINX/95.7% VBISX). Note however that if you’re using VFINX and VBISX you probably wouldn’t want to go higher than 90% VBISX, as doing so sacrifices considerable expected returns while reducing volatility very little (if at all). The green curve is in between the first two, with minimum volatility at 12.7% VFINX/87.3% VBIIX. I would not recommend going any higher than 80% VBIIX, though, from an expected returns/volatility standpoint. Reducing Volatility Beyond the Critical Point What do you do if you want to further reduce volatility after reaching your portfolio’s critical point? I see two reasonable options: Allocate some of your portfolio to treasuries (e.g. 10-year US treasury bonds). Swap for a shorter duration bond fund. Let’s go back to the first two-fund portfolio, VFINX/VBLTX. Suppose we’re at 25.8% VFINX/74.2% VBLTX and we recognize that we’ve reached minimum volatility. We would like to reduce volatility to one-fourth that of VFINX (the leftmost dotted line in the previous figures, at 0.298). We can’t do it with all of our assets allocated to VFINX or VBLTX. Let’s consider option (1). Allocating some of your portfolio to cash would pull the red curve down and to the left. But if you’re going to have cash, you may as well get some interest on it. So instead of cash let’s say we generate risk-free returns on whatever percentage we pull out of our VFINX/VBLTX portfolio, from investing those assets in US treasuries for example. The next figure shows the expected returns vs. volatility curves for various allocations to a risk-free investment that returns 1.5% annually. (click to enlarge) To clarify, the highest curve the same as we saw before; the next highest is 10% receiving risk-free 1.5% annual returns, and the remaining 90% split to VFINX/VBLTX in 10% increments; and so on until the lowest curve (which you can barely see), which is 90% risk-free 1.5% annual returns, and the remaining 10% split to VFINX/VBLTX in 10% increments. The first curve to extend to a volatility of 0.298 is the one with 40% allocated to the risk-free investment. For this portfolio, we would have to allocate the remaining 60% of our assets to 30% VFINX/70% VBLTX, to achieve an expected return of 0.0226% with volatility of 0.298%. Now let’s consider option (2). The next figure is the same as the last one, but with the curves for VFINX/VBIIX and VFINX/VBISX included. (click to enlarge) Interestingly, swapping VBLTX for VBISX lets us reach a volatility of 0.298 with a mean daily return slightly higher than that reached with VFINX/VBLTX and 40% risk-free. A 24.7% VFINX/75.3% VBISX portfolio has means returns of 0.0232%. A natural question is how the risk-free rate affects whether strategy (1) or (2) is better. For the Vanguard funds examined here, strategy (1) would always outperform strategy (2) if the risk-free rate was 4% or higher (i.e. rarely or never). Strategy (2) would always outperform strategy (1) if the risk-free rate was 0% (i.e. you held cash rather than treasuries). For risk-free rates between 0% and 4%, it really depends on the particular level of volatility you’re trying to achieve. Conclusions I think a lot of investors operate under the assumption that increasing exposure to bonds reduces volatility. But in fact there is often a point where further increasing exposure to bonds increases volatility and reduces expected returns. You don’t want to go past that point. To reduce volatility further than your two-fund portfolio allows, you can either allocate some of your assets to a risk-free investment, say US treasuries, or you can switch to a shorter duration bond fund. I favor the second strategy, as it tends to allow for greater expected returns and seems logistically easier to implement. More generally, I think it is very important to know where your portfolio is at in terms of the expected returns vs. volatility curve. You should have a good idea of how any potential change in asset allocation or choice of funds affects your portfolio’s characteristics.