NEW YORK (Bloomberg) - Currency wars, it turns out, may not be worth fighting right now.

While weaker exchange rates have at times throughout history helped stoke economic growth by making countries' exports cheaper, the benefits are becoming hard to find.

Nowhere is this more apparent than in developing nations, where currencies have slumped 24 per cent on average against the US dollar since 2011. Yet despite this, their annual export growth rate has actually slowed to 4 per cent in the past four years from 8 per cent during the previous decade, according to the CPB Netherlands Bureau for Economic Policy Analysis.

"The relationship between global growth and trade is breaking down in a way that we cannot apply the past relationship to predict the future," said Stephen Jen, a former International Monetary Fund economist who is co-founder of SLJ Macro Partners in London. "It now takes a bigger devaluation to have the same benefit."

Analysts point to a couple key reasons for the change: the fading effect of what were one-off factors that buoyed trade years ago, like the push by countries to cut tariffs; and China's shifting growth strategy.

As Beijing focuses more on the domestic economy and promoting growth in the service sector, China's demand for imports, while still growing, is slowing.

Speculation, nonetheless, is growing that countries across the world are stepping up efforts to weaken their exchange rates, a trend that then-Brazilian Finance Minister Guido Mantega famously labeled "currency wars" back in 2010. More than 20 countries have cut interest rates or taken other measures to ease monetary policy - moves that can curb demand for their currencies - since the start of the year.

All 31 major currencies fell against the dollar since the beginning of 2014, with 10 of them dropping more than 20 per cent, including the ruble, Norwegian krone and euro.

Not all countries, of course, are trying to weaken their currencies. The ruble, krone and even the real, for that matter, have tumbled in part because of the decline in prices for their commodity exports.

The International Monetary Fund did a recent study that illustrates why these currency declines are providing less help to a country. For every 1 per cent increase in global economic output, trade rose 0.7 per cent between 2008 and 2013. That's down from 1.5 per cent in seven years through 2007 and 2.2 per cent between 1986 to 2000, according to the IMF.

While the benefits of a weaker currency are harder to detect, it doesn't mean they've disappeared completely.

One quick way to see that is by looking at how investors bid up European stocks after the euro fell to a 12-year low against the dollar, a sign that they anticipate some economic gains to follow. The Stoxx Europe 600 Index has gained 16 per cent this year, compared with a 0.1 per cent increase in the Standard & Poor's 500 benchmark.

The decline of the euro "is obviously helping the European area a lot," said Jim O'Neill, former chairman of Goldman Sachs Asset Management. "But it's not creating extra demand. It's just at the expense of somebody else through competitiveness."

Yet a currency decline isn't a panacea. In Brazil, the real's 48 per cent plunge since 2011 has failed to buoy Latin America's biggest economy. Analysts in a central bank survey last week predict its gross domestic product will shrink 0.8 per cent this year, the worst recession since 1990. The currency's drop instead pushed annual inflation to a decade-high 7.7 per cent in February, prompting the central bank to raise rates even as the economy sputters.

"Currency depreciation is the easiest way out, at least for the short term, but it will lead to severe consequences if it's not accompanied by structural reforms," said Shweta Singh, a London-based economist at Lombard Street Research.