They have good reason. Millennials have grown up with dire warnings that Social Security will be exhausted by the time it is their turn to use it. They came of age in the worst downturn since the Great Depression, so they are no strangers to economic insecurity.

But there’s a paradox to thrift: Saving, even if virtuous on an individual level, can cause economic trouble en masse. If ambitious cash stockpiling were to catch on, it could exacerbate secular stagnation, a term that the Harvard University economist Lawrence H. Summers repopularized to describe the low-growth, low-inflation state of many advanced economies.

When consumers save a big portion of their income, they are not spending as much on dinners out, movie nights and cars. Businesses respond by investing less in equipment and technology, and productivity stalls. Bosses are unwilling to pay their workers more for the same output, and weak pay gains further restrain spending.

Retirement saving behavior is not the only driver causing economic torpor and lower rates. Inequality has left a small number of people with more money than they can realistically spend. Slower labor force growth and more iterative technological improvements could also have an impact.

The lower interest rates that result from high and unequal saving might sound great — think cheaper mortgages — but they leave economies vulnerable to shocks. In the United States, for example, rates are now in a range of just 1.5 percent to 1.75 percent, leaving the Fed room for about six quarter-point rate cuts in a downturn. Headed into the last recession, rates topped 5 percent.

Fed officials think mass bond-buying and promises to keep rates low for longer can give them power to fight a slump. But the jury is out on whether such alternatives will add enough ammunition to make up for lost room on interest rates.

Even Ben S. Bernanke, a former Fed chair with an optimistic take on the central bank’s ability to prop up the economy in a downturn, says officials could end up in a tight spot if rates drop substantially lower.