Economists are good at measuring the past but inconsistent at forecasting future events, particularly recessions. That’s because recessions aren’t caused merely by concrete changes in the markets. Beliefs and stories passed on by thousands of individuals are important factors, maybe even the main ones, in determining big shifts in the economy.

That is likely to be the case again, whenever we next endure a global recession. Worries that a big downturn might be imminent seem to have abated, but they still abound. In April, for example, the International Monetary Fund reported in its World Economic Outlook that while very modest growth is likely this year, the world economy was in a “fragile conjuncture.”

It is therefore worth asking what actually sets off a real global recession. Most discussions focus on leading indicators — statistics about economic variables that have preceded recessions. While these kinds of correlations can sometimes be useful in forecasting, they provide little understanding of why major changes are taking place. Leading indicators don’t usually address ultimate causes, nor do econometric models that try to predict events.

In fact, it’s instructive to remember that global recessions have usually begun suddenly and been a real surprise to most people. As I have argued in this column and with George A. Akerlof in “Animal Spirits” (Princeton 2009), such events can largely be ascribed ultimately to contagious stories of wide significance. Basically, global recessions tend to begin when newly popular narratives reduce individuals’ motivation to spend money. Psychology matters a great deal.