As emerging markets have slowed along with the world economy over the past few years, their exporters have struggled to chase after global demand. In principle, a general depreciation in their currencies, of the sort that accelerated sharply this week, could help.

Yet, there is little reason to believe this will happen. Not only is the depreciation as likely to be a consequence of economic weakness as a cure, but the benefit of competitiveness will most probably be outweighed by falling confidence and rising burdens of debt.

Part of the problem is that falling prices of oil and other commodities have been more bad than good for emerging economies as a whole. Commodities exporters have lost more than importers have gained, says David Lubin, head of emerging markets economics at Citi Research.