Natural Rate of Interest

The natural rate of interest is the theoretical real interest rate at which the economy’s desired saving equals desired investment, leaving no upward or downward pressure on inflation. It is perhaps the most elusive concept central banks try to estimate, because no economist can observe it directly—only infer it from inflation outcomes, growth patterns, and asset prices.

The Wicksell inheritance

The natural rate of interest entered economic thought via Knut Wicksell, the Swedish economist who in the 1890s asked a simple question: at what interest-rate would there be no inherent pressure for prices to rise or fall? His answer—a rate where voluntary saving exactly matched the economy’s desired investment—became foundational. If the central bank’s policy rate falls below this natural rate, savers have less incentive to defer consumption, investment demand rises, and producers face stronger demand relative to supply. Inflation creeps up. Conversely, a policy rate above the natural rate dampens both borrowing and spending, easing inflation pressure downward.

For over a century, this logic has anchored monetary policy theory. Yet Wicksell himself conceded that no one could measure his rate precisely. The problem persists today: the natural rate is not a dial on a dashboard. It cannot be quoted in financial markets or read off a spreadsheet. Central bankers must estimate it, and their estimates shift as data arrives and models evolve.

Why the natural rate matters to policy

The practical reason for caring about the natural rate is straightforward: it tells policymakers whether their current policy is loose, tight, or neutral. When the Federal Reserve sets its policy rate at, say, 2%, that answer depends entirely on what the Fed believes the natural rate to be.

Suppose the Fed’s economists estimate r-star at 3%. A policy rate of 2% is loose—below equilibrium—so demand will tend to strengthen, wages and prices will accelerate, and the Fed should raise rates. Now suppose a revision to the data persuades the same economists that r-star is actually 1%. Suddenly that 2% policy rate is tight, restraining growth and potentially pushing inflation below target. The policy rate is unchanged, but its stance has flipped simply because the estimate of the invisible rate changed.

This asymmetry—where small revisions to an unobservable quantity can flip policy conclusions—is one reason central banks publish forecasts of the natural rate and update them regularly. The Fed, for instance, includes estimates in the materials for its policy committee meetings.

How economists estimate the unobservable

Because r-star cannot be measured directly, economists use several indirect approaches.

Retrospective inference works backward from inflation outcomes. If inflation has been stable for years at roughly the central bank’s target, and growth has been near trend, then policy rates in that period were probably close to the natural rate. But this method is slow: it takes years for inflation to respond fully to a rate change, and by then, structural factors (demographics, productivity, technology) may have shifted the natural rate itself.

Model-based estimation uses macroeconomic models to solve for r-star as the long-run real rate consistent with full employment and stable inflation. Different models yield different answers, often ranging from 0.5% to 3% depending on assumptions about productivity growth, time preference, and discount rates. Central banks often average multiple approaches and report a range rather than a point estimate.

Market-based signals come from inflation-protected securities (TIPS), swap rates, and survey expectations of future inflation and growth. If inflation expectations are anchored and real yields on long-term bonds are 1.5%, that number is a clue about where markets expect the natural rate to settle. Of course, markets can be wrong, and asset prices reflect many forces beyond the natural rate.

The drift of r-star over decades

One striking pattern in the data is that estimated natural rates have drifted lower in the decades since the 1980s. Through the 1990s and 2000s, Federal Reserve staff estimates of the long-run natural rate hovered around 2.5% to 3%. By the 2010s, after the financial crisis, estimates fell to 1% or lower. Some economists argued the world had entered a “secular stagnation”—a period of persistently weak demand, weak investment, and weak growth—that had pushed the natural rate down and left central banks struggling to stimulate a sluggish economy.

Potential explanations include ageing populations (which boost saving), declining productivity growth, rising inequality, and a global glut of savings chasing limited investment opportunities. Each shifts the equilibrium, lowering the rate at which supply and demand for loanable funds would naturally balance.

During the pandemic and the recovery, however, estimates began to rise again. Supply-chain disruptions, fiscal stimulus, and supply-side constraints pushed inflation sharply higher, prompting questions about whether the natural rate had risen back toward pre-crisis levels, or whether temporary shocks had simply created a brief disconnect between policy rates and r-star.

Uncertainty and the margin for error

The single most important thing to understand about the natural rate is that it is an estimate surrounded by uncertainty bands, not a fact. The Federal Reserve publishes not a single number but a range. Academic studies produce conflicting results. If two equally competent economists can disagree by 1% or 2% on r-star, then policy decisions based on estimates can miss the mark substantially.

This uncertainty does not paralyze central banks—they set policy anyway, using the best estimates available—but it does counsel humility. A central bank that believes it is running a mildly stimulative policy (rates below r-star) might actually be running a neutral or mildly restrictive policy if r-star is higher than estimated. This margin for error is one reason policymakers look not just at their preferred estimate of the natural rate but also at multiple indicators: inflation outcomes, unemployment, wage growth, asset prices, and financial conditions.

The difference from the neutral rate

Do not confuse the natural rate with the neutral interest rate. The natural rate is a long-run real equilibrium; it is where the economy would settle if never disturbed. The neutral rate is a practical policy concept—the short-term interest-rate that neither stimulates nor restrains the economy once inflation is already at the central bank’s target. They are related but serve different purposes in monetary-policy discourse.

See also