The Taylor Rule and Policy Reaction Functions

Central bankers don't set interest rates by gut feel alone, but they also don't follow a single rigid formula by law. What they do have is a family of simple rules of thumb — reaction functions — that map economic conditions to a suggested policy rate, and the best-known of these is the Taylor rule, proposed by economist John Taylor in 1993. Markets use it constantly, not because central banks are obligated to follow it, but because it gives everyone a common benchmark for asking "is policy currently loose or tight relative to what the economy seems to call for?"

The idea

The Taylor rule says the policy rate should equal a baseline "neutral" real rate, plus adjustments for two things: how far inflation is running above its target, and how far output is running above or below the economy's normal capacity (the "output gap"). In words: raise rates when inflation is above target or the economy is overheating, cut them when inflation is below target or the economy is running slack. The original version weighted both inflation and the output gap roughly equally, but real-world central banks and economists use many variants with different weights and different measures of "normal."

A concrete example

Suppose the neutral real rate is 2%, target inflation is also 2%, current inflation is 3% (one point above target), and the output gap is zero (the economy is neither overheating nor slack). A simple Taylor rule with equal weights on the inflation gap would suggest a nominal policy rate of roughly 2% (neutral real rate) + 3% (current inflation) + 0.5 × 1% (the inflation gap) ≈ 5.5%. If the actual policy rate sitting in markets is only 4%, commentators would describe policy as "behind the curve" relative to the rule — looser than the rule's prescription — which is exactly the kind of comparison that shows up constantly in market commentary and central bank speeches.

What this means in practice

No central bank mechanically plugs numbers into the formula and sets rates accordingly — real policy weighs financial stability, credibility, and judgment calls the rule can't capture. But the rule matters anyway because markets and economists use deviations from it as a shorthand for gauging policy stance, and because a central bank that strays far from what simple rules suggest, without clearly explaining why, tends to face more scrutiny and volatility in how markets price its next move. For a rates trader, "where is policy relative to the Taylor rule" is a standard, quick sanity check before digging into a central bank's actual stated reasoning.