If that is the story – if the economy really has grown below trend of late – we might expect some rise in the unemployment rate to emerge before much longer. But this story still needs to explain the strength of tax receipts. Or maybe both productivity and output growth really have slowed, as the published estimates suggest.
The question then would be why productivity slowed so much. One hypothesis we hear from time to time is that average productivity might be reduced by adding the workers with the lowest productivity after a long expansion.