IT IS perhaps not surprising that the worst-performing major currency in the world this year is the Turkish lira. Many emerging-market currencies have taken a battering since the election in November of Donald Trump raised expectations of faster monetary tightening in America and sent the dollar soaring. But the lira has many other troubles to contend with, too: terrorist bombings, an economic slowdown, alarm over plans by the president, Recep Tayyip Erdogan, to strengthen his powers, and a central bank reluctant to raise interest rates to defend the currency. It has plunged to record lows. According to the Big Mac index, our patty-powered currency guide, it is now undervalued by 45.7% against the dollar.

The Big Mac index is built on the idea of purchasing-power parity, the theory that in the long run currencies will converge until the same amount of money buys the same amount of goods and services in every country. A Big Mac currently costs $5.06 in America but just 10.75 lira ($2.75) in Turkey, implying that the lira is undervalued.

However, other currencies are even cheaper. In Big Mac terms, the Mexican peso is...Continue reading