But that explains why firms didn’t cut wages when unemployment was high. How does it explain why they won’t raise wages now that unemployment is low again?

OK, here’s my theory about the brontosaurus, I mean, about wages. What employers learned during the long slump is that you can’t cut wages even when people are desperate for jobs; they also learned that extended periods in which you would cut wages if you could are a lot more likely than they used to believe. This makes them reluctant to grant wage increases even in good times, because they know they’ll be stuck with those wages if the economy turns bad again.

This hypothesis also explains something else that’s been puzzling me: widespread anecdotes about employers trying to attract workers with signing bonuses rather than higher wages. A signing bonus is a one-time cost; a higher wage, we now know, is more or less forever.