The strenuous efforts of monetary policies over the past decade need to be supplemented by fiscal expansion, especially if the global economy is slowing. But in the longer term, we need a better understanding of why private-sector investment has lost its mojo.

The Bank for International Settlements (BIS), the central bankers' club in Basel, Switzerland, recently conducted an in-depth evaluation of the unconventional monetary policies that have become the norm in many countries since the 2008 financial crisis. It should come as no surprise that the resulting report, compiled by a committee of central bankers reviewing their own past performance, finds little to fault. That is fine; the financial system was saved, after all. But a more important question is whether, and to what extent, unconventional policies remain relevant in the post-crisis world.

Back in 2008, the primary task for policymakers was to prevent the financial sector from collapsing and stabilise the economy. Conventional policies proved insufficient, and it became clear that other measures were needed to address the financial meltdown directly. Chief among these was quantitative easing, in which the central bank buys up assets in order to inject liquidity into the financial system.

In November 2008, at the...