Anatomy of a Rate Differential

The anatomy of a rate differential runs four layers deep: the gap between two central banks’ rates today, the gap markets expect, the gap after inflation, and the risk mood that decides whether any of it counts.

Most traders stop at the first layer because it is the easiest to look up. The pair is usually moving on one of the others. How to work out the basic gap is in What Is an Interest Rate Differential?

Today’s gap is already in the price

If the Fed pays 5% and the ECB 3%, every bank, fund and broker knows it. It is in EUR/USD and in the swap you pay or earn. Knowing it tells you nothing the price does not.

What moves a pair is news that changes the gap people expect over the coming months. A central bank can leave its rate untouched and still move its currency, by signalling that future rates will be higher or lower than markets thought. The Fed’s dot plot is one of those signals.

Two-year yields show the expected gap

A two-year government bond yield reflects roughly where markets expect the policy rate to average over the next two years. So the gap between two countries’ two-year yields is the market’s own running estimate of the differential to come.

An invented example. Both central banks sit at 4%, so the policy gap is zero. But one country’s two-year yield is 4.6% and the other’s 3.4%, because markets expect the first bank to hold and the second to cut. The expected gap is 1.2 points while the policy gap still reads zero.

Then a jobs report comes in above the forecast in the first country, and its two-year yield rises 0.15 points while the other stays flat. The expected gap is now 1.35. That change is what the currency reacts to, not the policy rates, which did not move at all.

Does the record agree?

Partly, and much more in some years than others. Take each month since 1999 and set the change in EUR/USD and USD/JPY beside the change in two gaps: the one between the central banks’ policy rates, and the one between two-year government yields.

From 2022 to September 2026, changes in the two-year gap lined up with 37% of the month-to-month variation in EUR/USD and 51% in USD/JPY. Changes in the policy-rate gap lined up with less than 1% in both.

It has not always been that strong. From 2008 to 2021 the two-year gap’s share was about a fifth (24% for EUR/USD, 19% for USD/JPY). From 1999 to 2007 it was small, 2% and 8%. The policy gap’s share never went above 5% in any of those stretches.

Those are squared correlations of month-end changes, worked out from the Fed’s exchange rates and Treasury yields, the ECB’s deposit rate, the Bank of Japan’s overnight call rate, the Bundesbank’s two-year Bund yield and Japan’s Ministry of Finance two-year yield. They describe the past, and the shares have already moved a lot from one period to the next.

Interest rate differential by currency pair shows the policy and two-year gaps side by side, and Major forex pairs: prices and fundamentals puts them beside each pair’s last price.

Subtract the inflation

A headline rate is a nominal rate. What a saver keeps after rising prices is the real rate, roughly the nominal rate minus expected inflation. Money that stays in a currency for months or years cares about the real rate.

Two invented central banks, both at 4%:

• Currency A has expected inflation of 2%, so a real rate of about +2%.
• Currency B has expected inflation of 5%, so a real rate of about −1%.

The nominal gap is zero. The real gap is three points in favour of A. A currency with a lower headline rate can still pay more in real terms, and draw the patient money.

Real interest rates by currency subtracts the latest headline inflation, year on year, from each currency’s policy rate and two-year yield. That is inflation as reported, standing in for the expected figure.

Which part of the curve is moving

A differential exists at every maturity. The policy rate sets the very front, two-year yields carry the next couple of years, ten-year yields the decade. Short-term traders and carry positions watch the front. Reserve managers and pension funds weigh the long end.

When the two disagree, slow down. A two-year gap that widens while the ten-year gap stays flat says markets expect the banks to drift apart soon and come back together later. That reads more like a repricing over a few months than the start of a long trend.

Where the gap gives way

On 31 July 2024 the Fed held at 5.25 to 5.50% (FOMC statement) and the Bank of Japan raised its rate to around 0.25% (Bank of Japan statement). The dollar still paid about five points more than the yen.

In early August, after a weak US release, the BIS found that FX carry trades were hit hard as leveraged positions were cut (BIS Bulletin 90). A five-point gap did not keep people in the trade once they were rushing to close it. The full story is in Carry Trade Guide: How It Works and Why It Unwinds.

In calm markets the gap tends to lead. In a scare, money moves to what the market treats as a safe-haven currency, whatever that currency pays.