Unveiling the Hidden Costs: Why Your Mortgage Bond ETF Might Be Lagging Behind
The Unexpected Financial Gap: MBS vs. Treasury Bonds in a Falling Rate Environment
In 2020, a hypothetical investment of $300,000 in the iShares MBS ETF (MBB) would have resulted in a deficit of $9,630 compared to an equivalent investment in the iShares U.S. Treasury Bond ETF (GOVT). This stark difference is particularly noteworthy given that both funds consist of government-backed bonds and operated within a period of sharply declining interest rates. While Treasury bonds capitalized on the market rally, mortgage funds experienced a drag on their gains as homeowners actively refinanced their mortgages.
Beyond Minimal Fees: The True Expense of Mortgage Bonds
While the iShares MBS ETF (MBB) boasts a minimal net expense ratio of just 0.04%, equating to a mere $120 annually on a $300,000 investment, this seemingly small cost belies a much larger financial impact. The actual burden on investors emerges from the distinct behavior of mortgage bonds when interest rates fall. For instance, in 2020, MBB yielded a 4.08% return, whereas GOVT delivered 7.29%. This performance gap, translating to $9,630 on a $300,000 principal, underscores that the primary cost isn't in management fees but in the structural dynamics of mortgage-backed securities during rate shifts. Even when compared to shorter-maturity Treasury bonds like the iShares 3-7 Year Treasury Bond ETF (IEI), MBB still lagged by $8,610, indicating that while maturity differences play a role, mortgage prepayments are a significant contributor to this disparity.
The Silent Erosion of Returns: How Mortgage Refinancing Caps Investment Gains
The mechanism behind this performance gap is rooted in how mortgage bonds react to refinancing. When interest rates decline, homeowners with higher existing mortgage rates are incentivized to refinance. This action leads to the premature repayment of their original loan at its face value. For an ETF like MBB, which holds bonds backed by these mortgages, this means that a bond that should be appreciating in value due to falling rates is instead retired. The fund then receives cash, which it must reinvest at the new, lower prevailing rates, effectively capping the potential upside. In contrast, Treasury bonds do not offer such an early repayment option, allowing them to continue appreciating in value as rates fall. This phenomenon is often termed "negative convexity" in bond markets, implying that the upside potential is limited while the downside risk remains.
Evidence from Fund Income: A Clear Reflection of Prepayment Impact
The operational impact of these prepayments is clearly visible in the income distributions of mortgage bond funds. For example, MBB's monthly distribution per share saw a notable decrease from $0.255997 in February 2020 to $0.191048 by November 2020, further plummeting to $0.018858 in October 2021. While specific figures for the portion of this decline attributable to prepayments are not separately detailed, the consistent pattern aligns precisely with scenarios where principal returns are reinvested at lower yields. Furthermore, the "government-backed" label, often perceived as a comprehensive safeguard, primarily offers protection against homeowner defaults, not against the financial consequences of early mortgage repayments.
The Double-Edged Sword: Rising Rates and Extension Risk
Conversely, when interest rates begin to climb, the dynamics reverse, presenting a different set of challenges for mortgage-backed securities. Higher rates lead to a decrease in refinancing activity, as homeowners are motivated to retain their existing lower-rate loans for longer durations. This situation extends the average maturity of the mortgage bonds within the fund, precisely at a time when longer-duration bonds are most sensitive to price declines from rising rates. This phenomenon is known as "extension risk"—mortgage bonds become shorter when investors prefer them longer, and longer when shorter would be advantageous. With the 10-year Treasury yield reaching 4.98% by October 29, 2026, any subsequent rate decreases could reintroduce the prepayment challenges observed in 2020.
Alternative Options: Similar Flaws Despite Lower Fees
Investors seeking alternatives might consider the Vanguard Mortgage-Backed Securities ETF (VMBS), which offers a slightly lower expense ratio of 0.03%, or $3 per $10,000 invested. While VMBS has shown slightly better long-term performance compared to MBB, for example, gaining 12.79% over ten years versus MBB's 12.2%, it holds the same type of agency mortgage bonds. Consequently, it exhibited similar underperformance in 2020, returning only 3.76%. Therefore, switching to VMBS primarily reduces fees but does not eliminate the fundamental drag caused by mortgage refinancing. Treasury funds such as GOVT or IEI, on the other hand, completely bypass prepayment risk, though this benefit comes at the cost of giving up the typically higher yields that mortgage bonds offer as compensation for taking on such risks.
Re-evaluating Portfolio Roles: What Mortgage Bonds Mean for Retirement Investors
For retirement investors who hold MBB or similar mortgage bond funds, it's crucial to reassess their intended role within the portfolio. If these investments are positioned as a "safe" proxy for Treasuries, expected to rally when rates fall and provide a buffer during stock market downturns, the 2020 experience provides a clear, data-driven answer. Mortgage-backed securities behave distinctly from pure Treasury bonds, and their inherent characteristics mean that the performance gap often widens precisely in the years when bond investors anticipate their fixed-income holdings to perform optimally. Understanding this behavior is essential for aligning investment choices with specific financial goals.
