Bond funds do not behave like the bonds inside them

Most explanations of how a bond fund differs from a bond stop at the point where it starts to matter. This one carries on.

The maturity that never comes

A single bond has a fixed maturity date on which the issuer repays the principal, assuming the issuer is still solvent by then. A bond fund holds many bonds, selling them as they approach maturity and buying new ones, so the portfolio itself never matures. This means a fund cannot offer the certainty of receiving a specific amount on a specific day, which an individual bond can.

What the fund offers instead is diversification across many issuers and a duration that stays roughly constant through time. People who bought a bond fund expecting the certainty of a bond are the ones most surprised when its price falls.

Duration is the number that matters

Duration measures how sensitive a bond portfolio is to a change in interest rates, and it is expressed in years. A fund with longer duration falls further when rates rise and gains more when they fall, all else being equal.

The useful part is this: this is arithmetic rather than sentiment, and the figure is published on essentially every bond fund fact sheet. Two bond funds carrying similar labels can have very different durations and therefore behave very differently in the same conditions. Comparing duration is far more informative than comparing yield when deciding what a bond holding will actually do for a portfolio.

Why a fall can be followed by higher income

When rates rise, existing bonds fall in price, and the fund reinvests maturing proceeds into new bonds paying the higher rate. Over a period broadly related to the fund's duration, that higher income offsets the initial price fall for anybody who keeps holding.

For most people, this is why a bond fund is less alarming for a long-term holder than the immediate price movement makes it look. It is equally why a holder who sells during the fall converts a temporary price effect into a permanent loss. None of this predicts rates, which nobody does reliably; it describes the mechanism rather than forecasting the outcome.

Credit risk is a separate dial

Government bond funds from stable issuers carry mainly interest rate risk, while corporate and high-yield funds add the risk of issuer default. Credit risk tends to appear at the same moments as equity market falls, which undermines the defensive purpose bonds were bought to serve. A fund holding lower-quality credit can therefore behave more like a diluted equity holding than like a stabiliser.

On an ordinary week, the fact sheet publishes a credit quality breakdown, and reading it is the quickest way to see which dial has been turned.

How much credit risk belongs in the defensive part of a portfolio is a genuine allocation decision rather than a fund detail.

Yield figures and what they mean

Distribution yield describes what has recently been paid out, while yield to maturity estimates the return if the holdings were held to maturity. These two can differ substantially, and the forward-looking figure is generally the more useful one when assessing a fund.

Neither is a promise, because the portfolio turns over continuously and both figures move as bonds are bought and sold. Chasing the highest published yield reliably leads towards longer duration, weaker credit quality or both at once. Yield describes the holdings rather than being a feature you can select independently of the risks that generate it.

If that does not fit your week, it is not a failure of willpower.

Where individual bonds still make sense

A known liability falling due on a known date can genuinely be matched by an individual bond in a way no fund replicates. That precision comes at the cost of concentration in a single issuer, unless enough separate bonds are bought to spread it. Buying individual bonds also involves spreads that are frequently wide for private investors compared with institutional dealing.

For most people building a general-purpose defensive allocation, the diversification of a fund outweighs the certainty being given up. Which is appropriate for a specific liability is a question for regulated advice rather than for a general article.

The takeaway

Read the duration and the credit breakdown. Those two numbers describe what a bond fund will do far better than its name.

The version you keep doing is the version that works.