The idea of America needing a big wall to keep out Mexicans is the political equivalent of aluminum: unattractive but recyclable. It’s not surprising then that Donald Trump wants to build a “great, great” wall to stem the flow of illegal immigration.

His platform might be fuzzy, but as the first position paper on his website clearly lays out, “There must be a wall across the southern border.”

But really, a big wall to keep Mexicans out is both ineffective and unnecessary.

Big walls don’t work. Sociologists have found that increased U.S. border security increases the number of undocumented immigrants. Getting over the border once is expensive, so doing it twice is unlikely. And that leads to the worst possible scenario: immigrants who have come to the U.S. “illegally” can’t find a job and now can’t leave either. We have managed to create a weird type of involuntary freeriding.

After all, as good as big walls are at keeping immigrants out, they’re just as good at sealing them in.

In theory, we could just deport all of the undocumented immigrants that are sealed in. But this is unrealistic. American Action Forum, a right-leaning think-tank, calculated that it would take 20 years and between $400 and 600 billion to deport all undocumented immigrants and prevent future undocumented immigrants from entering. That’s infeasible.

Increased border security leads to decreased freedom of movement. This is just one reason why 195 economists, lawyers and professors, among other professionals, signed the Open Borders Manifesto, which advocates removing state boundaries. The logic is that when people can cross from country to country more freely, they can travel to where their labor is actually needed. A 2011 paper in the Journal of Economic Perspectives found that removing labor migration barriers would increase global GDP by at least 50 percent.

But you don’t need to be an economist or wish to remove borders altogether, to realize that it’s a bad idea to keep people trapped in a country where they can’t find a job. More freedom of movement, not less, is the solution.

Further, if you look at some of the big walls throughout history and around the world today, it’s not clear why we need one. When I think of a really big wall that kept people out, I think of the Berlin Wall. But this worked mostly because of the amount of guards and a much smaller area to cover than the U.S.-Mexico border. And again, the Berlin Wall was built to keep people in, but I don’t think that’s the goal of Trump’s wall.

Of course, there’s the Great Wall of China, erected for military and defense reasons. The current biggest and most expensive walls are also defensive in purpose: Israel’s long-standing border wall with Palestine or Saudi Arabia’s new border wall with Iraq and parts of Yemen, for example. But wait, are we in a military conflict with Mexico?

No. We have a pretty nice relationship with Mexico — a $1.5 billion per day trading relationship. But it wasn’t always this way.

In order to find the justification to build a big wall to keep Mexicans out, you have to take a time machine and go all the way back to the 1910s. That’s when there were actual soldiers invading our borders. In 1916, for example, a famous Mexican military general, Pancho Villa, commanded an army that overran and raided a town in New Mexico, took over a U.S. cavalry camp and forced President Woodrow Wilson to send 75,000 troops to fight Villa’s army. But it’s not 1916 anymore. Mexicans aren’t invading American cities. Even the most at-risk demographic — undocumented Mexican men without high school diplomas — are three times less likely to be incarcerated than their native-born peers.

In short, we are much more likely to see migrant laborers come over the border than armies. We don’t need a big wall. We need more freedom of movement.

So why does the big wall keep invading discussions? Think of aluminum. Aluminum isn’t pretty, but it’s useful because it can be endlessly recycled. Similarly, the attractive characteristic of the big wall proposition isn’t its truth, but its enduring reusability to the conservative media and lawmakers.

After all, just as aluminum won’t disintegrate, the (manufactured) need for a big wall won’t just dissolve. Both are simply reprocessed for further use.