Interest rate differential

Topic: Balance of Payments and Exchange Rates · NCERT: Class 12, Ch 6 "Open Economy Macroeconomics"

Meaning

Interest rate differential is the gap between the interest rates of two countries, for example India's bond rate minus the US bond rate.

• It matters because money flows towards the country that pays more interest. That flow changes the demand for its currency.
• In the short run, this is one of the main forces that move the exchange rate.
• Rule (Class 12): if capital can move freely, a rise in home interest rates tends to make the home currency appreciate (gain value).
• Differential = i_home − i_foreign, where i is the interest rate.

Explanation

How it moves the exchange rate

• The chain from a rate gap to a currency move:
• Country B pays more interest than country A.
• Investors move their funds from A to B.
• To do this, they sell A's currency and buy B's currency.

• More demand for B's currency makes it appreciate. More selling of A's currency makes it depreciate.

• Worked example (NCERT):

• Bonds pay 8% in country A and 10% in country B. The gap is 2 percentage points.
• Funds leave A and go to B.

• A's currency depreciates and B's currency appreciates.

• Why this works in the short run: money is an asset. People hold a currency for the return it earns, and not only to buy goods. So capital flows, expectations and interest rates move the rate quickly. Trade flows matter more over the long run.

What makes the effect stronger or weaker

• Free capital movement: the rule assumes that money can move freely between countries. If capital controls (government limits on moving money in or out) block these flows, the effect is weaker.
• Expectations: investors compare the extra interest with the change they expect in the exchange rate. If they expect the high-interest currency to fall, the extra interest may not be worth it.
• Changes on either side: the gap changes when the home central bank changes its rate, and also when the foreign central bank changes its rate.
• Example: the home rate stays the same, but the US Fed raises its rate. The gap narrows. Money may leave the home country.

Interest rate parity (IRP): the deeper theory

• Interest rate parity says that the interest gap between two countries equals the expected change in their exchange rate. Once that holds, no risk-free arbitrage is left. Arbitrage means making a sure profit from a price gap.
• Covered IRP: the investor fixes the future exchange rate with a forward contract (a deal today to swap currencies at a set rate on a future date). Here, forward premium ≈ interest differential.
• Exact formula: F = S × (1 + i_home) / (1 + i_foreign)
• F is the forward rate and S is the spot rate (today's rate).

• Worked example: S = ₹83/$. India pays 7% and the US pays 4%.

  • F = 83 × 1.07 / 1.04 = ₹85.39
  • Shortcut: the forward premium ≈ 7% − 4% = 3%. So F ≈ 83 × 1.03 = ₹85.49.

• Uncovered IRP: the investor has no forward contract and carries the currency risk. Here, expected depreciation ≈ interest differential.

• Example: if India pays 3% more than the US, the market expects the rupee to fall about 3% a year against the dollar.

Carry trade: the differential used for profit

• Carry trade: borrow in a currency with low interest rates, such as the yen, and invest in higher-yield assets somewhere else.
• Example: borrow yen at about 0% and invest in dollar assets at 5%.

• Risk: if the yen rises 5% against the dollar, the whole gain is wiped out.

• The 2024 unwind (unwind means traders closing these positions in a rush):

• The Bank of Japan raised rates on 31 July 2024, earlier than markets expected [4].
• Weak US jobs data on 2 August 2024 added to the panic [4].
• The dollar–yen interest gap narrowed, and the yen rose sharply [4].
• Investors rushed to repay their yen loans. Yen carry trades were far larger than positions in any other currency [4].
• Japan's Nikkei fell 12% in one day, its biggest one-day move since 1987 [4].
• Markets recovered quickly. By mid-August the yen was steady near 140 per dollar [4].

In India

• Who sets the home rate: the RBI sets the repo rate (the interest rate at which the RBI lends money to banks for a short time). This rate shapes India's side of the differential.
• Repo rate and the rupee:
• When the RBI cuts the repo rate:
  • The India–foreign interest gap narrows.
  • Foreign money may flow out.
  • The rupee weakens, and imports such as oil cost more in rupees.

• When the US Fed raises its rate, the gap narrows in the same way, even if the RBI does nothing.

• Taper tantrum (2013): after 22 May 2013, the rupee swung sharply. Markets feared that the US Fed would slow its bond buying, which would raise US yields and pull money back to the US [3].

• Capital flows are not fully free: the rupee is not fully convertible (it cannot be freely exchanged for foreign currency for every purpose). So the NCERT rule works only partly for India.
• RBI response to swings: India runs a managed float. The market sets the rate, but the RBI says it intervenes only to curb excessive volatility (sharp swings in the rate). Its tools are spot deals, forwards and swaps, including "sell-buy" swaps [2].
• Global spillover: the 2024 yen carry-trade unwind spread to global markets, including India.

Don't confuse with

• NCERT rule vs uncovered IRP: under the NCERT rule, a higher home interest rate makes the home currency appreciate now, because money flows in. Under uncovered IRP, the higher-interest currency is expected to depreciate later by about the size of the gap. The first is about today's jump. The second is about the expected path afterwards.
• Covered IRP vs uncovered IRP: covered IRP uses a forward contract, so there is no currency risk (forward premium ≈ differential). Uncovered IRP has no cover, so the investor carries the risk (expected depreciation ≈ differential).
• Interest rate differential vs income effect: a higher home interest rate tends to appreciate the home currency. Higher home income raises imports and tends to depreciate the home currency.
• Interest rate differential vs currency speculation: the differential works through the extra interest an asset earns. Speculation works through an expected rise in the currency's price itself, and it can fulfil itself when many people act on the same belief.

Prelims Hooks

• Interest rate differential = the gap between interest rates in two countries. A rise in home interest rates → the home currency tends to appreciate. This assumes free capital movement.
• NCERT example: 8% in A and 10% in B → funds move to B → A's currency depreciates and B's currency appreciates.
• Covered IRP: forward premium ≈ interest differential. F = S × (1 + i_home) / (1 + i_foreign).
• Uncovered IRP: expected depreciation ≈ interest differential. There is no forward cover.
• Carry trade: borrow in a low-interest currency and invest in a high-yield one. BoJ hike on 31 July 2024 → yen carry-trade unwind → Nikkei fell 12% in one day [4].
• Trap: capital controls weaken the differential's effect on the exchange rate. They do not reverse it.

Mains Points

• Impossible trinity and the RBI's choices:
• With free capital movement, the RBI cannot fully control both the interest rate and the exchange rate.
• If it cuts the repo rate to support growth, capital may flow out, the rupee may weaken, and imported inflation from oil can rise.

• So monetary policy has to watch the interest gap with the US and other economies, and not only domestic inflation and growth.

• Global rate shocks reach India through the differential:

• The 2013 taper tantrum [3] and the 2024 yen carry-trade unwind [4] show how rate moves abroad can shake the rupee and Indian markets.

• The managed float, with RBI action only against "excessive volatility" [2], is India's buffer against these shocks.

• Short-term money is a mixed blessing:

• A large, positive interest gap draws in debt money that can leave quickly.
• That money funds the current account deficit (the gap when a country's payments for imports, services and income abroad exceed its earnings from them), but it can reverse suddenly.
• This supports a careful, step-by-step approach to capital account openness and a strong stock of forex reserves.

Related concepts

• Foreign exchange market
• Foreign exchange
• Foreign exchange rate
• Demand for foreign exchange
• Supply of foreign exchange
• Flexible exchange rate
• Currency depreciation
• Currency appreciation
• Currency speculation
• Interest rate parity

Read more

• The foreign exchange market and short-run exchange-rate determination