In December, Donald Trump and congressional Republicans passed large tax cuts, without bothering to pay for them. Less than two months later, they reached a budget deal with Democrats that increased government spending by hundreds of billions of dollars. With the U.S. economy at approximately full employment, was stimulus of this scope sound fiscal policy? Any Economics 101 student knows the answer is no.

It would be easy to dismiss this as just another case in which Trump rejected expert opinion—whether legal, scientific, or economic. But it actually runs deeper. It’s a classic example of what I call the Lamppost Theory: Politicians use economics the way a drunk uses a lamppost—for support, not illumination. Trump just made it worse.

It is widely but falsely believed that economists have enormous influence over economic policy. Economists control the Federal Reserve, right? Actually, that’s no longer the case. Unlike his four predecessors, Jerome Powell, who became the new chair of the Fed in February, is a former investment banker, not an economist. But doesn’t the Council of Economic Advisers still sit in the White House, with direct access to the president? Well, yes, it does, but presidents often ignore it, and Trump didn’t appoint a CEA chair for months.

The truth is that politicians were leaning on lampposts long before Donald Trump. The problem is long-standing and systemic, and it crosses party lines. It’s as if economists and politicians come from different civilizations that don’t understand one another. The late Arthur Okun, who was LBJ’s chief economic adviser, observed almost 50 years ago that, “On a number of issues, a bipartisan majority of the [economics] profession would unite on the opposite side from a bipartisan majority of Congress.” Apart from the fact that there are no longer any bipartisan majorities in Congress, those words ring true today.

Politicians, for better or worse, decide their positions on political criteria and then sometimes seek economists to bless those positions. (Trump often omits that last step.) Of course, the fact that decisions are made politically in a democracy is hardly shocking. Neither is it wrong. The real problem is that good economics often makes bad politics, and good politics of-ten makes bad economics. Since politics rules, economic policy often goes astray.