Foreign exchange has won broad acceptance as a legitimate asset class in recent years, despite the fact it is a zero sum game.

Unlike more established asset classes such as equities, bonds or property, currencies as a whole cannot rise in value – gains by some currencies must be cancelled out by losses by others. In investment parlance, foreign exchange has a beta of zero.

This has led investors to seek particular strategies that can, in theory at least, produce consistent positive returns. The best known of these is the carry trade, the strategy of taking long positions in currencies backed by high interest rates and shorting low-yielding currencies, which does appear systematically to produce positive returns, albeit with periodic stomach-churning reversals along the way.