One reason the Federal Reserve is likely to cut interest rates this week is that inflation is running below its 2% target. New research shows why getting it higher has proved so difficult: Many of the prices consumers pay don’t respond to the strength or weakness of the economy.

For decades, mainstream economists have seen inflation as determined by slack—that is, spare capacity—in labor markets and the broader economy. Too much slack should cause lower inflation; too little should drive up prices. This is captured in the Phillips curve, which shows an inverse relationship between unemployment and inflation.

Recent studies have shown prices in some sectors—such as housing—do indeed rise faster when growth is in full swing, unemployment low and markets frothy. But a large chunk of the economy, from health care to durable goods, appears insensitive to rising or falling demand.

A paper published last month by economists James Stock of Harvard University and Mark Watson of Princeton University found prices accounting for nearly half of the Fed’s preferred inflation gauge, the personal-consumption-expenditures price index, don’t respond to changes in economic activity. In 2017 economists at the Federal Reserve Bank of San Francisco found such “acyclical” goods and services made up a whopping 58% of that index.

The Fed influences inflation by lowering rates to spur demand or raising them to curb it. The new research suggests that to lift overall inflation the Fed may have to stimulate larger price increases in sectors where the Phillips curve still exists to compensate for subdued inflation in those where it doesn’t.