Why are deflationistas like me worried about the possibility of deflation? In a word, history.

Back when I learned macroeconomics — this was when dinosaurs roamed the earth, we performed simulations using stone axes, and students still learned about fiscal policy — one thing we were taught was that if you plot unemployment against inflation, the economy tends to go through clockwise spirals: high unemployment leads to falling inflation, then inflation stabilizes and maybe rises again once unemployment comes down. All of this was to be understood in terms of a Phillips curve in which actual inflation at any point in time depends both on the unemployment rate and on expected inflation, and expected inflation gradually adjusts in the light of experience.

These clockwise spirals were really clear in cases where a severe recession produced a spike in unemployment and disinflation. Here’s the slump of the mid-1970s:

And here’s the slump of the early 1980s:

By the way, in both pictures I use core inflation over the previous year; here’s why. Also, notice that there are occasional jinks and jogs in these pictures, since this is economics and data are far from perfect. The point is to focus on the big picture

So now we’re experiencing a slump that’s deeper and longer than either of those episodes; and we came into it with a much lower inflation rate than either previous experience. Are we seeing another clockwise spiral? Yes:

How can you look at this record and not conclude that deflation is a very real risk? I have no idea where the complacency of many at the Fed comes from.