A bond is a loan you make, usually to a government or a large company, in exchange for regular interest and the return of your money on a set date. That is the whole idea. Everything else — yields, duration, credit ratings — is detail layered onto that simple promise.
Yet bonds confuse people because their prices move, and move in ways that feel backwards. Understanding why turns them from a mystery into a tool.
Why bond prices move at all
If you hold a bond to maturity and the borrower does not default, you get exactly what was promised. But if you want to sell early, the price depends on today’s interest rates. When new bonds pay more, your older, lower-paying bond is worth less; when new bonds pay less, yours becomes more valuable. Price and prevailing rates move in opposite directions.
The longer a bond’s remaining life, the more sensitive it is to this effect. That sensitivity has a name — duration — and it is the single most useful thing to understand about a bond fund.
The real job: ballast
In a portfolio, high-quality bonds are not there to make you rich. They are ballast. In many downturns, when stocks fall, investors move money into safe government bonds and their prices hold up or rise. That partial offset is what lets a mixed portfolio fall less than a pure-stock one.
It is not a guarantee — there are periods when stocks and bonds fall together. But over a lifetime of investing, the tendency of quality bonds to steady the ship when equities are stormy is what earns them their place.
Matching bonds to your horizon
The rough rule is that money you need soon belongs in shorter, safer bonds, while money you will not touch for decades can afford more stocks and less bond cushioning. As retirement approaches, many people gradually add bonds precisely because they have less time to recover from a crash.
You do not need to pick individual bonds to do this. A low-cost, broadly diversified bond fund gives you thousands of loans in one holding, with the messy details handled for you.
The ballast claim, examined
The standard description of bonds as ballast rests on the observation that they have frequently risen when equities fell, which cushions the overall portfolio. This has been true in a number of significant episodes and it is a tendency rather than a rule, which is an important distinction that gets lost in the retelling.
The mechanism behind the tendency is that severe equity declines have often coincided with economic weakness, which has often prompted lower interest rates, which raises bond prices. When that chain holds, bonds do exactly what the ballast description promises.
When the chain does not hold — most notably when the problem is rising inflation rather than economic weakness — bonds and equities can fall together, and have. Anyone relying on bonds for protection should understand that they protect against one category of bad outcome well and against another category not at all.
Why not simply hold cash instead
If the purpose is stability, the obvious question is why not use cash, which does not fall at all. The answer is that the two do different jobs and the difference is worth being precise about.
Cash is genuinely stable in nominal terms and typically earns less than bonds over long periods, which makes it expensive to hold in size for decades. Bonds accept some price variability in exchange for a higher expected return, and crucially they have the potential to rise during equity declines, which cash does not. That potential is what makes them useful for rebalancing.
The rebalancing point is the strongest practical argument. A portfolio holding something that has risen while equities fell has more to sell into the decline, which is exactly when buying equities is most valuable. Cash provides the same function without the appreciation, which is a weaker version of the same benefit at a lower long-run cost. Most sensible portfolios hold both, for different purposes.
Matching bonds to a known future need
There is a second, quite different use for bonds that has nothing to do with portfolio ballast: matching a known liability to a known date. If you know you need a specific sum at a specific time, a bond maturing at that time delivers it with a degree of certainty no other instrument offers.
This is how institutions with defined future obligations use them, and the logic scales down. Someone with a known large expense in seven years — a school fee, a planned purchase, a mortgage balance falling due — can address it with an instrument that matures then, rather than by holding a general portfolio and hoping.
The requirement for this to work is holding to maturity, which means individual bonds rather than a fund, since a fund never matures. This is one of the clearest cases where the distinction between the two structures genuinely matters rather than being a technicality.
How the equity portion changes what bonds should do
The appropriate bond holding depends on what it is sitting alongside, which is why a single recommendation cannot be right for everyone. A portfolio with a large, volatile equity component needs its stabilising portion to be genuinely stable, which argues for shorter duration and the highest credit quality.
A more conservative portfolio, where the equity portion is smaller, can afford more variability in the bond portion because the overall exposure is lower. Someone in that position may reasonably accept longer duration in exchange for higher expected return.
The error to avoid is reaching for yield within the stabilising portion, which is a common and quiet way of increasing risk while appearing to reduce it. A portfolio whose safe half consists of long-duration lower-quality bonds has considerably more risk than the allocation percentages suggest, and the risk shows up at precisely the moments the allocation was designed for.
What happened when the ballast failed
It is worth being concrete about the failure case, because it happened recently enough to be within the memory of anyone reading this. In a period of sharply rising rates driven by inflation, longer-dated bonds experienced substantial declines at the same time as equities fell, and portfolios built on the assumption of an offsetting relationship did not get one.
Two lessons follow. The first is that the correlation between the two is not fixed; it varies with the underlying economic conditions and can turn positive. The second is that duration determined almost everything about how badly a bond holding was affected, which is why the earlier point about checking that single number matters.
The response is not to abandon the asset class, which would be the performance-chasing error described elsewhere on this site applied in reverse. It is to hold the appropriate duration for the job, to hold sufficient cash for the scenarios where nothing else helps, and to understand that no arrangement of financial assets protects against every category of bad outcome simultaneously.
Deciding how much, without a formula
Rather than a rule tied to age, a more useful approach starts from the decline you could tolerate without selling. Estimate what an equity-heavy portfolio would do in a severe fall, apply that to your actual balance, and ask honestly whether you would hold.
If the answer is no, the bond and cash portion needs to be larger, and the increase should be sized so that the answer becomes yes. This is a more demanding exercise than picking a percentage from a table and it produces an allocation you can actually live with, which is the only kind that survives.
The other input is the horizon of the money and the stability of the income supporting it. Both of these have been discussed elsewhere on this site and both matter more than age. The combination — what you can tolerate, when the money is needed, and how secure the income is — determines the answer more reliably than any single-variable rule. None of this is financial advice, and the appropriate allocation is specific to the person holding it.
Whose bonds, and in which currency
Two choices sit underneath any bond allocation and both are frequently made by default. The first is whose debt you are holding: your own government, other governments, or corporations. The second is what currency it is denominated in, which for the stabilising portion of a portfolio matters more than people expect.
A bond holding in a foreign currency introduces exchange rate movement, which can easily exceed the price movement of the bonds themselves. That variability defeats the purpose of a stabilising allocation, which is why international bond exposure in a retail portfolio is usually held with the currency risk hedged back to the home currency.
This is one of the few places where the more complicated-sounding option is the more conservative one. An unhedged foreign bond fund is a bet on currencies wearing the clothes of a safe asset, and it is a common holding among people who chose it on yield without noticing what else came with it.
The quiet role in a drawdown plan
Bonds acquire an additional function once a portfolio is being drawn on rather than added to, and it is the one that matters most at that stage. Holding an asset that is unlikely to have fallen sharply means withdrawals can come from it during an equity decline, leaving the equity portion untouched to recover.
This addresses the sequence problem described elsewhere on this site directly. A retiree drawing entirely from equities during a severe early decline locks in the loss permanently. One drawing from a bond and cash allocation during the same period does not, and the difference in outcomes over a long retirement is substantial.
The practical structure that follows is a small number of years of expenses held outside equities, replenished from the equity portion during good periods rather than bad ones. It is not complicated and it changes the risk profile of a drawdown considerably more than any adjustment to the overall allocation percentages does.
Reading a bond fund fact sheet
Almost everything you need to assess a bond holding sits on a single page and takes a few minutes to read. Four figures do most of the work, and knowing what they are removes the sense that this asset class is opaque.
Effective duration tells you how sensitive the price is to rate changes and is the single most predictive number for how the holding will behave. Average credit quality tells you what kind of borrower you are lending to and therefore how correlated the fund is likely to be with equities. Yield to maturity gives a rough indication of the return available from here if things go as scheduled.
The fourth is the ongoing charge, which matters proportionally more for bonds than for equities because the expected returns are lower. A charge that is a small fraction of an equity return can be a substantial fraction of a bond return, which is why cost discipline in this part of a portfolio deserves more attention than it usually receives.
Why this asset class is worth understanding at all
It would be reasonable to ask why an ordinary investor should engage with any of this, given that many people hold a single mixed fund that handles it internally. The answer is that the bond component is where most mixed portfolios differ from one another, and where most of the unexplained risk sits.
Two funds described identically in terms of their equity and bond split can behave very differently depending on the duration and credit quality inside the bond half. An investor who understands only the headline percentages has no way of anticipating that, and typically discovers it during the one period when it matters.
None of this requires expertise. It requires knowing that duration and credit quality exist, checking them once, and understanding roughly what each implies. That is an hour of effort, applied once, to the portion of a portfolio that is supposed to be the reliable part — which is a reasonable place to spend an hour. As with everything on this site, this is educational rather than advice.