Because of narrowing interest rate differentials, concerns about the world economic outlook and the possibility of intervention.

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However, in our interview, Mr Trichet gave the criticism short shrift, and categorically denied that he had ever pointed to the narrowing of interest rate differentials as a sign of the euro's success.

BBC: Interview with an ECB president

Because interest rate differentials are a big factor in currency moves, the DBV shorts the three currencies of G10 countries with the lowest rates and goes long the three with the highest rates.

FORBES: Trade Currencies Like Soros

David Woo, a currency strategist at Bank of America Merrill Lynch, says that markets are now moving on real (after inflation) interest rate differentials rather than the nominal gaps they used to heed.

ECONOMIST: Currencies

There's a lot more to quantitative investing than magic formulas and leveraged bets on teeny interest-rate differentials.

FORBES: Revenge of The Nerds

But volumes will fall because corporate customers will no longer need to hold accounts in several currencies, and banks will no longer be able to use the money market to profit from interest-rate differentials between euro-zone currencies.

ECONOMIST: The imminent launch of the euro is a threat to Europe��s banks