Purchasing Power Parity: Why Exchange Rates Ignore It for Years

Purchasing power parity is the idea that a basket of identical goods should cost the same in any two countries once you convert at the exchange rate. If a basket costs 100 dollars in the US and 90 euros in the euro area, the rate should be 0.90.

It is intuitive, economically sensible, and persistently wrong about what exchange rates actually do. Understanding why is more useful than the theory itself.

The reasoning behind it

The logic is arbitrage. If an identical good is cheaper in one country, buyers should purchase it there and sell it where it is dearer. That demand would push prices and the exchange rate toward equality.

For a single freely traded commodity, something like this does hold. Gold does not trade at wildly different prices in London and New York for long, because anyone could ship it.

The theory then extends that logic to the whole economy. That extension is where it breaks.

Why it fails in practice

Most things cannot be traded. This is the central problem. You cannot import a haircut, a flat in central Paris, a school place or a dental appointment. Non-tradable services are a large share of what people spend money on in developed economies, and no arbitrage mechanism connects their prices across borders at all.

Trade has friction. Shipping, tariffs, quotas, insurance and regulatory approval all drive wedges between prices. A good must be mispriced by more than the total cost of moving it before anyone acts.

Taxes differ. Sales taxes and duties vary enormously and are baked into consumer prices.

Baskets differ. People in different countries buy different things. Comparing "the same basket" requires constructing something that nobody actually purchases.

Capital flows dwarf trade flows. This is the decisive one for exchange rates. Currency markets are driven far more by investment and interest rate differentials than by trade in goods. Money chasing yield moves rates in ways that trade arbitrage cannot offset.

The Big Mac Index

The best-known illustration takes one product available in a standardised form worldwide and compares its price.

Its appeal is real: it sidesteps the difficulty of defining a basket, and everyone understands the product.

Its limitation is equally real. A substantial share of the price is local rent, local wages and local taxes — none of which are traded internationally. So the index largely measures differences in local costs, which is exactly the thing PPP struggles with.

It is a teaching device, and a good one. It is not a valuation model.

What PPP is genuinely useful for

Its failure as an exchange rate predictor does not make it useless. It has two solid applications.

Comparing living standards. Converting national income at market exchange rates makes poorer countries look poorer than they are, because non-tradable goods are cheaper there. PPP-adjusted GDP per capita is a far better comparison of what people can actually buy, and it is why the IMF, World Bank and OECD publish PPP figures.

Very long-run anchoring. Over decades, currencies with persistently higher inflation do tend to depreciate. The relationship is loose, slow and swamped by other factors over shorter periods — but it is not nothing.

Why deviations persist so long

Deviations from PPP have lasted a decade or more, repeatedly and in both directions.

The reason is that nothing forces convergence. There is no arbitrage that closes the gap between the price of a flat in one country and a flat in another. Meanwhile:

• Interest rate differentials attract capital regardless of relative prices.
• Safe-haven flows move currencies for reasons unconnected to goods.
• Productivity differences justify permanently different price levels — richer countries have genuinely higher non-tradable prices, an effect economists call the Balassa-Samuelson relationship.

That last point matters: some of the deviation is not a mispricing at all. It is the correct reflection of different economies.

For anyone following currencies

PPP is not a trading signal. A currency 20% "overvalued" on PPP can stay there for years and become more so. Traders who shorted overvalued currencies on this basis have been early for long enough to be wrong.

Interest rate differentials dominate over the horizons anyone actually operates on. The two-year yield spread between two countries explains far more currency movement than relative price levels do.

Use it as context. A currency far from PPP tells you something about accumulated pressure. It tells you nothing about timing, and timing is the entire problem.

The bottom line

Purchasing power parity describes an equilibrium that markets drift toward over decades and ignore in between. It is genuinely valuable for comparing what people in different countries can afford, and genuinely poor at predicting where an exchange rate goes next.

The gap between those two uses is why it appears in every economics textbook and almost no trading strategy that works.

This article is educational and is not financial advice. Leveraged foreign exchange trading carries a high risk of loss.