Duration-Matching Liabilities
Duration-Matching Liabilities
The safest bond strategy is not to pick the highest-yielding fund, but to buy bonds that mature when you need to spend the money.
Key takeaways
- A liability is a specific amount of money you must pay on a specific date.
- Matching bond duration (or maturity) to liability dates eliminates timing risk and simplifies reinvestment decisions.
- Pension funds and institutional investors use liability-matching religiously; individual investors rarely do, but should.
- Mismatched duration exposes you to reinvestment risk: you may have to reinvest at lower rates.
- Liability-matching is the foundation of immunization strategy and the conceptual ancestor of all modern portfolio management.
What is a liability?
A liability, in this context, is a known, dated outflow of cash. Examples:
- You will retire in 15 years and need to withdraw £2,000 per month (£360,000 over that period).
- Your child will attend university in 8 years; tuition will be £30,000.
- You have a mortgage payment of £1,000 monthly for 20 years.
- A pension plan owes retirees £5 million annually for the next 30 years.
- A corporation must repay a bond at maturity in 5 years: £100 million.
Liabilities are facts. You do not negotiate with them or hope they vanish. The liability-matching principle is stark: if you know when and how much you must pay, buy bonds that generate cash at those exact times.
Why naive portfolio construction fails liabilities
Suppose you have £100,000 and need £10,000 each year for 10 years (a simple liability). A naive approach might be to hold a diversified bond fund with 5-year duration, reasoning "5 years is reasonable, and the fund is diversified." This approach has three problems.
First, in year 1, you withdraw £10,000. The remaining £90,000 is still invested. In year 2, market rates have moved, the portfolio's duration has shifted, and reinvestment conditions have changed. You cannot be sure the £10,000 withdrawal in year 2 will come from the portfolio—it might come from selling bonds at a loss if rates have risen.
Second, you have no guarantee that the fund will generate £10,000 of income in year 1, year 2, and so on. Bond funds distribute income (coupons) and capital gains, but the amount is unpredictable.
Third, you incur constant trading costs (bid-ask spreads, potential capital gains taxes) as you rebalance or withdraw.
A liability-matching approach inverts the problem. You say: "I need £10,000 in year 1, £10,000 in year 2, ... £10,000 in year 10. Let me buy bonds that mature on those dates." If you buy a 1-year bond yielding 4% with face value £10,417, it will pay you exactly £10,417 (principal plus coupon) in one year. For year 2, buy a 2-year bond with face value £10,203 yielding 4.5%. And so on for years 3–10.
This portfolio is perfectly matched to your liability. No reinvestment uncertainty. No market-timing risk. When each bond matures, you receive exactly what you need.
Pension funds and liability matching
Pension funds pioneered liability-matching in the 1970s and 1980s. A pension has a clear liability: it must pay retired workers and their beneficiaries for decades. In 2024, the UK's NHS pension scheme has a liability of hundreds of billions of pounds, spread across thousands of beneficiaries and maturity dates.
A pension manager constructs a bond portfolio (sometimes called a "liability-matching portfolio" or "LM portfolio") designed to generate cash flows matching expected beneficiary payments. The portfolio might hold:
- £500 million of bonds maturing in 2025
- £550 million maturing in 2026
- £600 million maturing in 2027
- And so on, through 2055+
As each maturity date arrives, the pension receives cash from maturing bonds and pays it to retirees. Reinvestment risk is minimal because the portfolio is explicitly built to match payouts.
This strategy works because pension liabilities are predictable. A 65-year-old retiree will likely live 20–25 more years; actuaries can estimate the expected payout. For an individual investor, liabilities are often predictable too—but investors rarely structure portfolios to match them.
Individual liability matching examples
Example 1: Saving for a child's education. Your 10-year-old will attend a 4-year university starting in age 18. You expect tuition to be £20,000 per year in nominal terms (adjusted for inflation). Rather than holding a generic bond fund, consider:
- Buy a 8-year bond (matures when year 1 tuition is due) with face value £20,500, yielding 4%.
- Buy a 9-year bond (matures when year 2 tuition is due) with face value £21,400, yielding 4.1%.
- Buy a 10-year bond (year 3 tuition) with face value £22,300, yielding 4.2%.
- Buy a 11-year bond (year 4 tuition) with face value £23,200, yielding 4.3%.
(Inflation assumptions embedded in the face values.) Each maturity provides exactly what you need. You sleep well; there is no "will I have enough?" anxiety.
Example 2: Bridging to retirement. You are 60 and plan to retire at 65, but your pension does not start until 67. You need to draw £30,000 per year from savings for years 65, 66, and 67. Rather than holding a 5-year bond fund and hoping it lasts, buy:
- A 5-year bond with face value £31,200 (matures at age 65).
- A 6-year bond with face value £32,500 (matures at age 66).
- A 7-year bond with face value £33,800 (matures at age 67).
Again, each maturity maps exactly to a need.
Example 3: Debt repayment. You have £200,000 in a variable-rate business loan at SONIA + 2%. You worry that rates will rise and you will struggle to pay interest. One solution is to liability-match: buy a bond ladder (2-year, 3-year, 4-year, 5-year bonds) whose coupon payments cover your interest costs. As rates rise, your bond coupons rise too (or you have locked in yields), providing a natural hedge.
The mechanics of liability matching
Liability-matching requires:
- Identify liabilities: List every known cash outflow, with date and amount.
- Choose bonds or build a ladder: For each liability date, select bonds that mature on (or near) that date.
- Size the position: The bond's face value plus coupon should cover the liability, with a small buffer for rounding.
- Hold to maturity: Do not sell early. Interest rate moves become irrelevant because you are not sensitive to prices—you will receive face value at maturity.
- Reinvest coupons: As the portfolio receives coupon payments before a maturity date, reinvest them in short-duration instruments (money market funds, savings accounts) to earn some return without adding duration risk.
For individual investors, a simplified version might involve:
- Buying a bond ladder (1-year, 2-year, ... 10-year bonds) and taking the cash flow as each matures.
- Holding some ETFs (like BND or AGG) and rebalancing annually to approximate a glide-path (shortening duration as a known liability date approaches).
- Using target-date funds, which automatically adjust duration and asset allocation toward a specific date.
When liability matching is imperfect
Perfect liability-matching assumes you know amounts and dates precisely. In reality:
- Inflation uncertainty: You know your child will attend university, but tuition inflation may be 2% or 5% per year. You will need to update your estimates.
- Longevity uncertainty: A retiree does not know if they will live to 85 or 95. Pensions use actuarial life expectancy; individuals must guess.
- Optionality: A liability might not be fixed. You might take an early pension, retire earlier, or delay a purchase. This fuzzes the date.
- Illiquidity: Individual bonds held to maturity are illiquid. You cannot easily exit if circumstances change.
For these reasons, even sophisticated investors do not match liabilities with 100% precision. Instead, they match the core liabilities (the highest-confidence future payments) with bonds, and hold more flexible instruments (equities, shorter-duration bonds, or a small emergency fund) for contingencies.
Duration versus maturity in liability matching
Duration and maturity are not the same. Maturity is the date a bond pays principal; duration is the weighted-average time to receive cash flows. For liability-matching purposes, maturity is more important than duration.
If you know you need £10,000 in 5 years, buying a 5-year zero-coupon bond (duration = 5 years, maturity = 5 years) is perfect. Buying a 10-year bond (maturity = 10 years, duration ≈ 7 years) does not match well; you will receive principal too late.
However, a bond with 5-year maturity and 4-year duration (e.g., a bond that pays high coupons and thus returns capital sooner) could create a mismatch if coupons are reinvested at lower rates than anticipated. To eliminate reinvestment risk entirely, zero-coupon bonds or very high-coupon bonds that return capital quickly are preferred.
Flowchart: Liability-matching decision
Next
Liability-matching is the foundation of immunization strategy, a more sophisticated approach that locks in a specific rate of return regardless of how interest rates move after your initial purchase. While liability-matching focuses on matching dates and amounts, immunization adds a guarantee: that your portfolio will grow to exactly the target value even if rates shift dramatically during your holding period.