It’s a sure thing that Donald Trump will spend much of his State of the Union boasting about the economy. So this seems like a good time for a refresher on some basic macroeconomics – and the reasons why the expansion of 2017, which continued the long expansion that began in 2010, is in no sense a justification for wildly optimistic growth projections looking forward.

As a reminder, the Trump Treasury department claims that tax cuts will pay for themselves because the economy will grow at almost 3 percent a year for the next decade. This growth projection didn’t come from any model; it was just pulled out of … well, you fill in the rest. But every time there’s a good quarter of growth, the usual suspects take time off from talking about deep state conspiracies to claim that the forecast is coming true. Why is this nonsense?

First, you need to know that quarter-to-quarter and even year-to-year growth rates are very variable. The economy grew at a 5 percent annual rate during much of the Carter administration (how many people know that?); it grew around 4 percent during the second Clinton administration: