IT IS more than two weeks since the Federal Reserve raised interest rates for the first time in over nine years, and the world has not (yet) ended. But it is too soon to celebrate. Several central banks have tried to lift rates in recent years after long spells near zero, only to be forced to reverse course and cut them again (see chart). The outcome of America’s rate rise, whatever it may be, will help economists understand why zero exerts such a powerful gravitational pull.

Recessions strike when too many people wish to save and too few to spend. Central banks try to escape the doldrums by slashing interest rates, encouraging people to loosen their grip on their money. It is hard to lower rates much below zero, however, since people and businesses would begin to swap bank deposits for cash or other assets. So during a really nasty shock, economists agree, rates cannot go low enough to revive demand.

There is significant disagreement, however, on why economies become stuck in this quagmire for long periods. There are three main explanations. The Fed maintains that the problem stems from central-bank paralysis, either self-induced or politically imposed. That prevents the use of unconventional monetary policies such as quantitative easing—the printing of money to buy bonds.

The intention of QE is to buy enough long-dated debt to lower long-term borrowing rates, thereby getting around the interest-rate floor. Once QE has generated a speedy enough recovery, senior officials at the Fed argue, there is no reason not to raise rates as in normal times.

If the Fed is right, 2016 will be a rosy year for the American economy. The central bank expects growth to accelerate and unemployment to keep falling even as it lifts rates to 1.5% or so by the end of the year. Yet markets reckon that is wildly optimistic, and that rates will remain below 1%. That is where the other two explanations come in.

The first is the “liquidity trap”, an idea which dates back to the 1930s and was dusted off when Japan sank into deflation in the late 1990s. Its proponents argue that central banks are very nearly helpless once rates drop to zero. Not even QE is much use, since banks are not short of money to lend, but of sound borrowers to lend to.

Advocates of this theory see only two routes out of the trap. The government can soak up excess savings by borrowing heavily itself and then spending to boost demand. Or the central bank can promise to tolerate much higher inflation when, in the distant future, the economy returns to health. The promise of higher-than-normal inflation in future, if believed, reduces the real, or inflation-adjusted, interest rate in the present, since money used to repay loans will be worth less than the money borrowed. Expectations of higher future inflation therefore provide the stuck economy with the sub-zero interest rates needed to escape the rut.

Governments pursued both these policies in the 1930s to escape the Depression. But when they reversed course prematurely, as America’s did in 1937, the economy suffered a nasty and immediate relapse. The liquidity-trap explanation suggests the Fed’s rate rise was ill-advised.

The American economy, after all, is far from perky: it is growing much more slowly than the pre-crisis trend; inflation is barely above zero; and expectations of inflation are close to their lowest levels of the recovery. If this view is correct, the Fed will be forced by tumbling growth and inflation to reverse course in short order, or face a new recession.

Stuck in a glut
 
There is a third version of events, however. This narrative, which counts Larry Summers, a former treasury secretary, among its main proponents, suggests that the problem is a global glut of savings relative to attractive investment options. This glut of capital has steadily and relentlessly pushed real interest rates around the world towards zero.

The savings-investment mismatch has several causes. Dampened expectations for long-run growth, thanks to everything from ageing to reductions in capital spending enabled by new technology, are squeezing investment. At the same time soaring inequality, which concentrates income in the hands of people who tend to save, along with a hunger for safe assets in a world of massive and volatile capital flows, boosts saving. The result is a shortfall in global demand that sucks ever more of the world economy into the zero-rate trap.

Economies with the biggest piles of savings relative to investment—such as China and the euro area—export their excess capital abroad, and as a consequence run large current-account surpluses.

Those surpluses drain demand from healthier economies, as consumers’ spending is redirected abroad. Low rates reduce central banks’ capacity to offset this drag, and the long-run nature of the problem means that promises to let inflation run wild in the future are less credible than ever.

This trap is an especially difficult one to escape. Fixing the global imbalance between savings and investment requires broad action right across the world economy: increased immigration to countries with ageing populations, dramatic reforms to stagnant economies and heavy borrowing by creditworthy governments. Short of that, the only options are sticking plasters, such as currency depreciation, which alleviates the domestic problem while worsening the pressure on other countries, or capital controls designed to restore monetary independence by keeping the tides of global capital at bay.

If this story is the right one, the outcome of the Fed’s first rises will seem unremarkable. Growth will weaken slightly and inflation will linger near zero, forcing the Fed to abandon plans for higher rates.

Yet the implications for the global economy will be grave. In the absence of radical, co-ordinated stimulus or restrictions on the free flow of capital, ever more of the world will be drawn, indefinitely, into the zero-rate trap.