On Monday Jeb Bush — or I guess that’s Jeb!, since he seems to have decided to replace his family name with a punctuation mark — finally made his campaign for the White House official, and gave us a first view of his policy goals. First, he says that if elected he would double America’s rate of economic growth to 4 percent. Second, he would make it possible for every American to lose as much weight as he or she wants, without any need for dieting or exercise.

O.K., he didn’t actually make that second promise. But he might as well have. It would have been just as realistic as promising 4 percent growth, and considerably less irresponsible.

I’ll get to Jeb!onomics in a minute, but first let me tell you about a dirty little secret of economics — namely, that we don’t know very much about how to raise the long-run rate of economic growth. Economists do know how to promote recovery from temporary slumps, even if politicians usually refuse to take their advice. But once the economy is near full employment, further growth depends on raising output per worker. And while there are things that might help make that happen, the truth is that nobody knows how to conjure up rapid productivity gains.

Why, then, would Mr. Bush imagine that he is privy to secrets that have evaded everyone else?

One answer, which is actually kind of funny, is that he believes that the growth in Florida’s economy during his time as governor offers a role model for the nation as a whole. Why is that funny? Because everyone except Mr. Bush knows that, during those years, Florida was booming thanks to the mother of all housing bubbles. When the bubble burst, the state plunged into a deep slump, much worse than that in the nation as a whole. Taking the boom and the slump together, Florida’s longer-term economic performance has, if anything, been slightly worse than the national average.

The key to Mr. Bush’s record of success, then, was good political timing: He managed to leave office before the unsustainable nature of the boom he now invokes became obvious.

But Mr. Bush’s economic promises reflect more than self-aggrandizement. They also reflect his party’s habit of boasting about its ability to deliver rapid economic growth, even though there’s no evidence at all to justify such boasts. It’s as if a bunch of relatively short men made a regular practice of swaggering around, telling everyone they see that they’re 6 feet 2 inches tall.

To be more specific, the next time you encounter some conservative going on about growth, you might want to bring up the following list of names and numbers: Bill Clinton, 3.7; Ronald Reagan, 3.4; Barack Obama, 2.1; George H.W. Bush, 2.0; George W. Bush, 1.6. Yes, that’s the last five presidents — and the average rate of growth of the U.S. economy during their time in office (so far, in Mr. Obama’s case). Obviously, the raw numbers don’t tell the whole story, but surely there’s nothing in that list to suggest that conservatives possess some kind of miracle cure for economic sluggishness. And, as many have pointed out, if Jeb! knows the secret to 4 percent growth, why didn’t he tell his father and brother?

Or consider the experience of Kansas, where Gov. Sam Brownback pushed through radical tax cuts that were supposed to drive rapid economic growth. “We’ll see how it works. We’ll have a real live experiment,” he declared. And the results of the experiment are now in: The promised boom never arrived, big deficits did, and, despite savage cuts to schools and other public services, Kansas eventually had to raise taxes again (with the pain concentrated on lower-income residents).

Why, then, all the boasting about growth? The short answer, surely, is that it’s mainly about finding ways to sell tax cuts for the wealthy. Such cuts are unpopular in and of themselves, and even more so if, like the Kansas tax cuts for businesses and the affluent, they must be paid for with higher taxes on working families and/or cuts in popular government programs. Yet low taxes on the rich are an overriding policy priority on the right — and promises of growth miracles let conservatives claim that everyone will benefit from trickle-down, and maybe even that tax cuts will pay for themselves.

There is, of course, a term for basing a national program on this kind of self-serving (and plutocrat-serving) wishful thinking. Way back in 1980, George H.W. Bush, running against Reagan for the presidential nomination, famously called it “voodoo economic policy.” And while Reaganolatry is now obligatory in the G.O.P., the truth is that he was right.

So what does it say about the state of the party that Mr. Bush’s son — often portrayed as the moderate, reasonable member of the family — has chosen to make himself a high priest of voodoo economics? Nothing good.