It can seem counterintuitive that rising interest rates, generally treated as a signal of a tightening or strengthening economy, are bad news for existing bond prices. The relationship is actually mechanical, not a matter of investor sentiment: a bond locks in a fixed interest payment when it's issued, and once broader interest rates move, that fixed payment either looks more or less attractive relative to what newly issued bonds are now offering — and a bond's market price adjusts to compensate for that gap.
A bond's payments are fixed; new bonds aren't
A bond is essentially a loan: the buyer lends money to the issuer, and the issuer promises to pay a fixed rate of interest (called the coupon) over the bond's life, then repay the original principal at maturity. That coupon rate is set when the bond is issued and doesn't change afterward, regardless of what happens to interest rates in the broader economy. If prevailing interest rates rise after a bond has been issued, newly issued bonds will offer a higher coupon rate than the older bond locked in at the previous, lower rate — which means the older bond, still promising its original fixed payment, has become comparatively less attractive to a buyer who could instead buy a new bond paying more.
The price has to fall to make the old bond competitive again
Because the older bond's fixed coupon payments can't change, the only way it can remain competitive with newly issued, higher-paying bonds is for its market price to fall — a lower purchase price means a buyer gets the same fixed future payments for less money upfront, effectively raising the return they earn on their investment (called the yield) to bring it back in line with what new bonds are offering. This produces the well-known inverse relationship between bond prices and interest rates: when rates rise, existing bond prices fall to keep their yields competitive; when rates fall, existing bonds with their now comparatively attractive fixed payments become more valuable, and their prices rise. The relationship isn't a matter of market mood or speculation, it's simply what has to happen mathematically for an existing bond's yield to stay aligned with whatever new bonds are currently offering.
What we're still unsure about
The mechanical, inverse relationship between bond prices and interest rates is a well-established, essentially arithmetic result in finance, not something genuinely disputed. What's more a matter of estimation than certainty is exactly how sensitive a specific bond's price will be to a given change in interest rates, since that sensitivity — a property called duration — depends on the bond's specific maturity length and coupon structure, and predicting how much a real bond's price will actually move in response to a real interest rate change requires applying that calculation carefully to the bond's specific terms, rather than assuming every bond responds to rate changes by the same amount.
This sits inside Bond Valuation & Interest Rate Risk, one of eight topics in Finance, one of five domains in Economics, one of seventeen subjects the app can quiz you on.