Haruhiko Kuroda, the governor of the bank of Japan said last week that

China should use capital controls to stabilize its currency.

Azerbaijan, Turkmenistan and Nigeria have all introduced or tightened capital controls this month,

formally much despise they may be making a comeback.

Policy makers often grapple with what is called the impossible trinity.

You can have your own monetary policy, you can have a floating exchange rate,

and you can have the free movement of capital.

But you can't have all three.

Often it's the free movement of capital that gets ditched first.

India brought in restrictions on foreign exchange markets in 2013,

and they were credited with helping to keep its currency stable.

Brazil used them temporarily on its fixed income market, apparently without much damage.

And in 2011, the international monetary fund published guidelines on how to use capital controls,

which was taken as quite a turnaround and as an endorsement.

Many investors and economists still dislike them because they can go so badly wrong.

Venezuela's dysfunctional currency regime is one example.

But capital controls are no longer the mark of the devil.

Christine Lagarde, the IMF's managing director was on the same panel as Mr. Kuroda

when he made his suggestion in Davos.

She didn't reject the idea and that's been taken as tacit approval.

Also this month Agustín Carstens, the governor of Mexico's central bank

said it was time for emerging markets' central bankers to become unconventional.

And China is already doing it.

It imposed restrictions on offshore bank accounts this month.

The fact is that the world's economic problems are becoming harder and harder to tackle

and policy makers are turning to whatever tools are in the tool box.