According to press reports, the Trump administration is basing its budget projections on the assumption that the U.S. economy will grow very rapidly over the next decade — in fact, almost twice as fast as independent institutions like the Congressional Budget Office and the Federal Reserve expect. There is, as far as we can tell, no serious analysis behind this optimism; instead, the number was plugged in to make the fiscal outlook appear better.

I guess this was only to be expected from a man who keeps insisting that crime, which is actually near record lows, is at a record high, that millions of illegal ballots were responsible for his popular vote loss, and so on: In Trumpworld, numbers are what you want them to be, and anything else is fake news. But the truth is that unwarranted arrogance about economics isn’t Trump-specific. On the contrary, it’s the modern Republican norm. And the question is why.

Before I get there, a word about why extreme growth optimism is unwarranted.

The Trump team is apparently projecting growth at between 3 and 3.5 percent for a decade. This wouldn’t be unprecedented: the U.S. economy grew at a 3.4 percent rate during the Reagan years, 3.7 percent under Bill Clinton. But a repeat performance is unlikely.

For one thing, in the Reagan years baby boomers were still entering the work force. Now they’re on their way out, and the rise in the working-age population has slowed to a crawl. This demographic shift alone should, other things being equal, subtract around a percentage point from U.S. growth.