The world's central banks are engaged in one of the great policy experiments in modern history: ultra-easy money. And, as the experiment has continued, the risk of failure — and thus of the wrenching corrections and deep economic dislocations that would follow — has grown.

In the wake of the crisis that began in 2007, policy rates were reduced to unprecedented levels, where they remain today, and measures were taken to slash longer-term rates as well. Nothing like it has ever been seen before at the global level, not even in the depths of the Great Depression. Moreover, many central banks' balance sheets have expanded to record levels, although in different ways and for different rationales — further underscoring the experimental character of the monetary easing now underway.

The risks implied by such policies require careful examination, particularly because the current experiment appears to be one more step down a well-trodden path — a path that led to the crisis in the first place.