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Hope never dies, particularly not when it can give the stockmarket a boost in uncertain times. When the financial crisis rolled out into a general economic downturn last year, the theory of "decoupling" raised its head, offering the prospect that China and other big emerging economies would be able to continue to grow in such a way as to keep the global economy afloat. That proved not to be the case as the BRICs (Brazil, Russia, India and China) felt the strain of sharply falling external demand and the tightening of capital flows.

But now it is back, buoyed largely by the not-as-bad-as-we-feared data coming out of China. Goldman Sachs, which first coined the BRIC...