The Fed was clearly overoptimistic about the economy’s prospects, as it has pretty consistently been for the past decade. It’s worth noting that throughout that whole period conservative critics of the Fed — the same people now backing Trump — attacked the institution for keeping interest rates too low, not too high. Still, it’s now clear that the attempt to normalize monetary policy was premature.

But the Fed’s premature rate hikes aren’t why the Trump tax cut is failing. How do we know that? Because all those boasts about why the tax cut would work miracles were based on a specific story about what is holding the U.S. economy back. And that story was and is all wrong.

The Trumpist theory — which was, I’m sorry to say, endorsed by conservative economists who should have known better — was that there was a huge pile of money sitting outside the U.S. that companies would bring back and invest productively if given the incentive of lower tax rates. But that pile of money was an accounting fiction. And the tax cut didn’t give corporations an incentive to build new factories and so on; all it did was induce them to shift their tax-avoidance strategies.

As Brad Setser of the Council on Foreign Relations points out, a casual glance at the data seems to suggest that American companies earn a lot of their profits at their overseas subsidiaries. But a closer look shows that the bulk of these reported profits are in a handful of small countries with low or zero tax rates, like Bermuda, Luxembourg and Ireland. The companies obviously aren’t earning huge profits in these tiny economies; they’re just using accounting gimmicks to assign profits earned elsewhere to subsidiaries that may have a few factories, but sometimes consist of little more than a small office, or even just a post-office box.

These basically phony profits then accumulate on the books of the overseas subsidiaries, rather than the home company. But this doesn’t affect their ability to invest in America: if Apple wants to spend a billion dollars here, it can always borrow the money using the assets of its Irish subsidiary as collateral. In other words, U.S. taxes weren’t having any significant effect in deterring real investment in the U.S. economy.

When Trump cut the tax rate, some companies “brought money home.” But for the most part this had no economic significance. Here’s how it works: Apple Ireland transfers some of its assets to Apple U.S.A. Officially, Apple Ireland has reduced its investment spending, while paying a dividend to U.S. investors. In reality, Apple as an entity has the same total profits and the same total assets it did before; it hasn’t devoted a single additional dollar to purchases of equipment, R&D, or anything else for its U.S. operations.

Not surprisingly, then, the investment boom Trump economists promised has never materialized. Companies didn’t use their tax breaks to invest more; mainly they used them to buy back their own stock. This in turn, put more money in the hands of investors, which gave the economy a temporary boost — although for 2018 as a whole, one of the biggest drivers of faster growth was, believe it or not, higher government spending.