LAST week's Free exchange column took a look at the ongoing economic discussion over "secular stagnation", or the argument that a chronic shortfall of investment relative to saving might be shackling the rich world with weak demand.

Before the financial crisis, excessive thrift in emerging economies may have played a role. In 2005 Ben Bernanke identified a “global saving glut” as the reason for low interest rates. Many emerging economies, particularly China, had rising current-account surpluses. They sent their surplus savings to the rich world, by building up large foreign-exchange reserves, mostly in the form of rich-world bonds. This drove up asset prices and fuelled housing bubbles. A new working paper from the National Bureau of Economic Research reckons that foreign capital flows to America drove down interest rates and accounted for as much as a third of the increase in house prices in the 2000s. But this explanation for economic stagnation in the rich world is difficult to square with today’s data. Global growth in foreign-exchange reserves slowed dramatically in 2013. Yet rich economies are still struggling while asset prices continue to soar. Another theory holds that high savings reflect a cramping of consumption due to rising inequality of incomes. The share of income earned by the top 1% began climbing in the early 1980s and now stands close to the record set in 1928. Rich households save more than poorer ones. A paper published this year by Barry Cynamon of the St Louis Fed and Steven Fazzari of Washington University in St Louis estimates that prolific saving by the top 5% has been suppressing demand since the mid-1980s. That squeeze was mostly offset by increased borrowing by the bottom 95%, they find. America and Britain, unlike Germany and Japan, saw rapid growth in private debt in the 2000s (see chart 2). But when the crisis forced households to deleverage, the underlying inequality-driven stagnation may have reasserted itself.

In working through the arguments about secular stagnation, two key points really stand out to me. One is that unless you think the entire world is suffering from the disease, secular stagnation implies that afflicted countries (which are saving more than they are investing) are exporting excess saving abroad and therefore running current-account surpluses. That very much seems to describe Germany (as Paul Krugman notes here) and Japan. It does not at all describe America or Britain.

Which takes us to the second point, which is that it really isn't clear whether there is more to "secular stagnation" than too-tight monetary policy. Maybe Germany has it. But Japan, which has looked like a secular stagnation economy for two decades suddenly seems less a candidate, and not because of any remarkable structural shifts in the economy.

The most straightforward story for a place like America is that various factors, from inequality to foreign reserve accumulation to demand for safe assets, have depressed nominal interest rates in America. And too many officials have interpreted low interest rates as reflecting sufficiently stimulative—"ultra-loose" is the common phrasing"—monetary policy. But you can't judge a monetary policy by nominal interest rates; you can't, you can't, you can't. So while it's interesting to talk about the potential causes of secular stagnation, such talk shouldn't be driving policy in places not called Germany.