Interest Rate Risk

Interest rate risk is the risk that a change in prevailing interest rates leaves a person or an institution worse off than before. The SEC states the bond version of it directly: "Interest rate changes can affect a bond's value. If bonds are held to maturity the investor will receive the face value, plus interest. If sold before maturity, the bond may be worth more or less than the face value."

That is one half of the phrase, and it is worth naming the other explicitly because both are in ordinary use. A holder of a fixed stream of payments, meaning a bondholder, is exposed to rates rising: their payments cannot increase, so the price of what they hold falls until it competes with newly issued alternatives. A payer of a floating rate, meaning a borrower on an adjustable-rate loan, is exposed to the same event in an entirely different form: nothing about their asset changes, and the payment goes up. The two are not different risks with one name. They are the two ends of the same arrangement, which is why one party can only be protected by the other accepting the exposure, and why the protected party pays for it.

Someone is always on the other side, and the price of the transfer is visible. A fixed-rate mortgage does not remove interest rate risk from the world; it moves it from the borrower to the lender, who carries it for the whole term and charges for doing so. An adjustable-rate mortgage leaves it with the borrower after the introductory period and prices the loan lower to reflect that. Neither structure is safer in the abstract. The question is which party can better absorb a rate move, and what the transfer costs.

For a bondholder, the size of the exposure is a number rather than an impression. How far a bond's price moves for a given change in yields is measured by its duration, and duration depends on maturity, coupon and yield rather than on maturity alone. Two bonds maturing on the same day can carry materially different exposure. A bond fund reports an average duration for the same purpose.

The exposure cannot be avoided, only shaped, and this is the part most worth understanding. A holder who buys long maturities to lock in today's rate accepts that the price of the holding will move a great deal if rates change. A holder who stays short avoids most of that price movement and accepts something else: every time a short holding matures, the money has to be reinvested at whatever rate prevails on that day, and nobody knows what that will be. That is reinvestment risk, and it is the mirror image rather than a smaller version. Rising rates hurt the first holder and help the second. Falling rates do the reverse. There is no maturity at which both problems disappear, and a portfolio holding cash is not exempt: cash is the shortest possible maturity and therefore carries the most reinvestment risk of all.

Holding to maturity is a choice of form, not an escape. A holder who keeps a sound issuer's bond until it is repaid never realizes a price loss, and the statement never shows one. What that holder does instead is spend the remaining years collecting a coupon below what the market is now paying. A bond bought at 4 percent when new bonds pay 6 percent is a decision, renewed every day it is kept, to earn less than the alternative. The cost is real, it is measurable, and it appears nowhere. This is also why an individual bond and a bond fund are less different than they look on this point: the fund prints the cost immediately as a lower share price, and the individual bond spreads it silently across the term.

Where rates come from, briefly. The Federal Reserve sets a target range for a very short-term rate and steers the market rate inside it, which strongly influences the short end. Longer yields are set by the market, and they reflect expectations about future short rates and about inflation, plus compensation for the uncertainty in both. So a bondholder's exposure is not only to what the Fed does next; it is to what the market comes to expect over the whole life of the bond, which is why long bonds can move sharply on days when no policy rate changed at all.

A change in prevailing rates changes what a newly issued loan or bond pays. Every existing fixed obligation is then compared against that new standard, and the comparison shows up either in a price or in an amount of income forgone. Floating obligations skip the price step and reset the payment instead.

A hypothetical example, showing both forms of the bondholder's exposure. Yusuf holds a $10,000 bond paying 4 percent, so $400 a year, with 8 years left to run. Comparable new bonds begin paying 6 percent.

If he sells. The bond has to compete with new bonds paying $600 a year, so its price falls until the two are comparable offers. Suppose its modified duration is 6.5: the estimated decline is 6.5 × 2 percentage points = 13 percent, so it sells for roughly $8,700 and Yusuf takes a $1,300 loss.

If he holds. He receives every promised dollar, and $10,000 comes back in eight years. He also collects $400 a year while the alternative pays $600, so he gives up $200 a year for 8 years, which is $1,600 of income he could have had. No statement records it, no tax form reports it, and it is the same exposure showing up in a different place.

Neither figure is the "true" cost, and the two are computed on different assumptions, but the pattern holds: holding a bond after rates rise converts a visible loss into an invisible one rather than into none.

Now the other side of the same event. Sanne owes $300,000 on an adjustable-rate mortgage. Rates rise by the same 2 percentage points at her next adjustment. Her balance does not fall in value, because she is the one who owes it; instead her interest cost rises by roughly $6,000 a year ($300,000 × 0.02) until she pays down principal or the rate resets again. Yusuf and Sanne experienced one event. It arrived as a price for one of them and as a bill for the other.

Pros (of managing it deliberately)

• It is measurable in advance. Duration turns a vague worry about rates into a number that can be compared across holdings.
• Because it has two forms, it can be chosen rather than merely suffered: maturities can be matched to when the money is actually needed.
• It is symmetrical, so the same exposure that costs a holder when rates rise pays them when rates fall.
• The trade between fixed and floating is priced openly in the loan market, so the cost of transferring the risk to a lender is visible before signing.

Cons (and limits of managing it)

• It cannot be eliminated. Avoiding price risk means accepting reinvestment risk, and holding cash maximizes the second.
• Duration is an approximation, and it says nothing about credit, so a rate-matched portfolio can still fail for a different reason.
• The cost of holding a below-market bond appears on no statement, which makes it easy to conclude that holding to maturity was free.
• Nobody can predict rate moves reliably, so positioning a portfolio for a forecast substitutes one risk for another.