The Federal Reserve can lower interest rates to zero, or even–as the European Central Bank has done–into negative territory. But it can’t make lenders lend or would-be borrowers borrow if they don’t want to. And it certainly can’t make a shopper open their wallet and spend if they’re more inclined to save.

Economist John Maynard Keynes is often credited for comparing these limits of monetary policy to “pushing on a string,” but the term actually predates him. Congressman T. Alan Goldsborough used the term during the congressional hearings on the Banking Act of 1935.

At any rate, it is a good metaphor. And it’s the problem facing the United States, Europe, Japan and even China. The Fed’s quantitative easing (“QE”) programs have run their course–at least for now–and America is slowly moving towards more “normal” monetary policy. Whether or not QE worked in the U.S. is a matter of debate. My view is that is was wildly successful is stoking a bubble in the stock market and in giving homeowners a refinancing windfall, but not much else. Credit growth is still very weak, and consumers are not as eager as to spend as they were before the 2008 meltdown. But while the U.S. has pulled back from its QE excesses, Japan and Europe are just getting started, and China is getting more creative as well. Thus far, none of this has amounted to much more than pushing on a string.

The Economist recently suggested that demographics were the cause of this secular stagnation.

I agree. Years of  working with Harry Dent taught me that a person’s age is the single biggest contributing factor in their spending decisions. Advertizers have understood this since the dawn of mass consumerism. You’re a lot more likely to see commercials for Viagra or life insurance during a baseball game than you are during a Twilight movie. But economists tend to ignore the role of demographics, focusing instead on big “macro levers” like interest rates and government spending.