One thing we can say for sure is that it is not the slightly weaker-than-expected retail sales that triggered the mayhem on Wall Street on Wednesday morning. Most US economic data have actually been quite strong in the month since Wall Street peaked on Sept 19.
So to find an economic rationale for the biggest stock-market decline since 2011, we have to consider two other explanations.
The first is the collapse of oil prices, down almost 30% since late June in response to Saudi Arabia's apparent decision to wreck the economics of US shale oil. Falling oil prices are generally beneficial for the world economy - and for most businesses outside the energy sector. But investors now fleeing from natural-resource stocks will take time to recycle their money into other industries, such as airlines, retailers and auto manufacturers. Until this rotation happens, the plunging oil shares, a process visible, drag down broad stock-market indices almost every day in the past two weeks, especially in the last hour of trading.
If falling oil prices were the main causes of the market setback, it would not be a big problem. There is, however, a far more worrying explanation: Europe. Not just the obvious weakness of the European economy, but the inability or unwillingness of European Union policymakers to agree on a sensible response.
Europe's economic weakness was already evident several months ago when the Ukrainian crisis and Russian sanctions broke the momentum of German industrial growth, which had been a rare bright spot in the continent's economic outlook.
But investors and business leaders were not too worried by the prospect of a sanctions-related slowdown in Germany because they assumed that Europe's politicians and central bankers would respond with stimulative policies similar to the ones that had pulled the US economy out of several "soft patches" in the past five years. Because of this confidence in policy stimulus, global and US stock markets were able to keep hitting new records in the summer, despite bad news from Europe.
For most of the period since 2008, Europe's miserable economic performance did not seem to bother investors - as long as the US economy was doing all right. Even at the height of the euro crisis, global stock-market performance has been more influenced by the gyrations of US economic statistics and Federal Reserve policy than by anything happening in Greece, Italy or the European Central Bank.
In the past few weeks, however, bad news from Europe seems suddenly to be having far more impact than the generally positive news from the United States, where economic growth is accelerating and expectations of interest rate hikes have been pushed back from next spring to September or beyond.
Why has this happened?
In previous columns, I have explained the lockstep gyrations of the US economy and global stock markets by the demonstration effects of US monetary and fiscal policy. Because the United States pioneered the policy responses to the 2008 economic crisis - quantitative easing, near-zero interest rates and unprecedented budget deficits - investors assumed that the success or failure of these policies in the U.S. economy today would eventually spread to the rest of the world.
When the US economy seemed to be moving toward a sustainable expansion, it seemed reasonable to suppose that the rest of the world would follow, with a lag of a year or two. When, however, US growth suffered an unexpected setback - as it did last winter and in the summers of 2011 and 2012 - investors and businesses would turn pessimistic around the world.
After all, if the United States was unable to pull convincingly out of recession after $3.5tn of quantitative easing, five years of near-zero interest rates and budget deficits worth 10% of gross domestic product, what hope could there be for other countries implementing half-hearted versions of the same program?
Once each of these U.S. growth scares turned out to be just a temporary aberration, bullish sentiment returned. Not only on Wall Street, but also in Europe and emerging markets, on the view that if monetary and fiscal stimulus were shown to be working in the U.S. economy, other governments and central banks would eventually follow similar policies and achieve similar results.
Now it appears that this linkage may have broken. The European Central Bank bitterly disappointed investors who had expected the bank to follow the Federal Reserve's example and announce dramatic monetary measures, combined with a convincing recapitalization of the European banking system, at European Central Bank President Mario Draghi's press conference on Oct 2.