Turkey's lira has taken a big hit this year, and it shows in the cost of a Big Mac

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 percent 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 the United States 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 undervalued by an enormous 55.9 percent against the greenback. This week it also plumbed a record low as Trump reiterated some of his campaign threats against Mexico. The peso has lost a tenth of its value against the dollar since November. Of big countries, only Russia offers a cheaper Big Mac, in dollar terms, even though the ruble has strengthened over the past year.

The eurozone is also prey to political uncertainty. Elections are scheduled this year in the Netherlands, France and Germany, and possibly in Italy. The euro recently fell to its lowest level since 2003. Britain's Brexit vote has had an even bigger effect on the pound, which has fallen to $1.21, a 31-year low. According to the Big Mac index, the euro and the pound are undervalued against the dollar by 19.7 and 26.3 percent, respectively.

Historically, this adjustment has tended to raise currencies' valuations against the dollar, so emerging-market currencies tend to look more reasonably priced. The Chinese yuan, for example, is 44 percent undervalued against the dollar according to our baseline Big Mac index, but only 7 percent according to the adjusted one. The deluxe Big Mac index has typically made rich-world currencies look more expensive. Because western Europeans have higher costs of living and lower incomes than Americans, the euro has traded at around a 25 percent premium against the dollar in income-adjusted burger terms since the euro's inception.