The bond fund I bought for the wrong reason

I'm down 0.81% on a position I still think was a good idea. Both things are true, and the gap between them is the whole post.

September 2025. I added IBGS (iShares € Govt Bond 1-3yr UCITS ETF) to a growth-heavy portfolio and wrote down two reasons at the time. One: a defensive buffer at all-time highs. Two, the reason that matters here: it would "capture ECB rate cuts while retaining liquidity." I expected capital gains if the ECB kept cutting.

That second reason was wrong twice over. Wrong about the direction the ECB would go, and wrong, separately, about how the instrument I'd chosen behaves. The second mistake is the useful one, because it's a mistake almost anyone holding a bond fund is exposed to without knowing it.

The number nobody looks up

A bond fund's price doesn't move on the coupon. It moves on duration, a figure in years that tells you roughly how sensitive the fund's price is to a change in interest rates. The rule of thumb: price change ≈ duration × the change in yield, sign flipped. Yields fall, price rises; yields rise, price falls.

IBGS has an effective duration of 1.65. A 100 basis-point move in yields (a "basis point" is one-hundredth of a percentage point, so 100 of them make one point) moves the fund's price by about 1.65%. A fund holding longer-dated bonds, with a duration of 7, moves by about 7% on the same 100bp swing.

I wanted to capture rate cuts. A short-duration fund is close to the worst instrument for that job. If you want to profit from falling rates, you buy duration: a long-dated bond fund, not a 1-3 year one. I'd picked the fund built to minimize the exact effect I said I was trying to capture.

The maturity structure compounds it. IBGS's underlying bonds mature, on average, in 1.71 years, so the fund constantly rolls maturing bonds into new ones at whatever yield is on offer that day. In a cutting cycle, that reinvestment happens at progressively lower yields, and the income stream starts shrinking within two years. Short duration in a falling-rate world gives you the worst of both sides: minimal price gain, plus a coupon that's already eroding by the time you notice.

The fund I bought to capture cuts was structurally incapable of doing that job well. My stated thesis and my instrument choice pointed in opposite directions, and I didn't catch it for months.

It worked anyway, for a different reason

The position was fine. That should make you uncomfortable, not reassured. During a three-month stretch when the growth side of the portfolio fell roughly 8.8%, IBGS stayed close to flat. That was never the rate bet; it was the ballast. Short-duration government bonds don't swing much in either direction. That's exactly why they do little for a rate-cut thesis, and exactly why they dampen a drawdown. Same property, opposite consequence, depending on what you're asking the fund to do.

I didn't predict what came next either. The ECB pivoted to hiking: its deposit rate went from 2.25% to 2.50% in September, the second hike of the new cycle, and the German 2-year Schatz has been auctioning around 3.27%, up from 2.85% before the pivot. I had no view on this; it surprised me as much as anyone. But it means the fund is now positioned correctly for the regime it's in, for the first time since I bought it. Low duration is a liability when rates fall and an asset when they rise: it limits the price hit from each hike and lets the portfolio re-lend into higher yields within about two years. The instrument that was wrong for my original thesis is right for the world I'm now in, for reasons that have nothing to do with why I bought it.

The arithmetic you can run yourself

The useful version of "duration × change in yield" is the one you flip around: how much can rates move against you before the position loses money?

Breakeven rate rise ≈ yield ÷ duration

For IBGS, with a yield around 3% and duration of 1.65, that's roughly 1.8 percentage points. Eurozone short rates would have to rise nearly two full points in a year before this position is in the red, and if they do, the underlying bonds are rolling into that higher yield within about two years anyway. For a typical aggregate bond fund, with a duration nearer 7, the same 3% yield buys a breakeven of only about 43 basis points. A single below-consensus inflation print could erase it.

Same yield. A four-fold difference in how much room you have before you lose money. Almost nobody holding either fund has looked up which side of that line they're on.

The sensitivity table makes it concrete. Carry is the yield you collect just for holding the bonds; it's the floor under the position even before you think about price:

The same logic run in reverse explains something that looks unrelated: why REITs and utilities, often called "bond proxies" because their income streams behave like coupons, get hit so hard in a hiking cycle. Their cash flows stretch out indefinitely, so their effective duration is very long, and unlike a bond fund there's no maturity date forcing a reset into the new, higher yield. A rate move that a short-duration bond fund absorbs and recovers from within two years hits a bond proxy permanently.

What I'd actually take from this

The comfortable lesson is that it worked out. I don't buy that as the real one. My stated reason for owning this fund was wrong in both direction and mechanism, and the position still did its job. That means the process was broken even though the outcome wasn't. Those two facts don't cancel out. A right outcome from a wrong process teaches you nothing if you never go back and find the error.

The number worth carrying forward is the effective duration of whatever bond fund you hold. It sits on every factsheet, and it's the single figure that decides whether the next rate move is a catastrophe or a raise. Multiply it by a plausible yield move and you know, before the fact, roughly what the fund will do. Divide the yield by it and you know how much room you have. Most people who own a bond fund have never run either calculation. It takes about thirty seconds once you know where to look.

Not investment advice: this is a description of one position and how I've come to understand it, not a recommendation.