"The wall will stop the drugs," Donald Trump promised in a recent interview with the Associated Press, reaffirming his simpleminded faith in the power of interdiction to prevent Americans from getting the psychoactive substances they want. Although drugs from Latin America are "coming around [border barriers] in certain areas," the president explained, "if you have a wall, they can't do it because it's a real wall." Thanks to the real wall, he said, "we'll stop all of it."

No, they won't. As Theresa Cardinal Brown explains in the May issue of Reason, "drug smugglers have already beaten Trump's wall" through a variety of evasive maneuvers, such as hiding drugs inside shipments going through the wall at legal points of entry, using tunnels to carry drugs under the wall, flying or catapulting drugs over the wall, and transporting drugs around the wall on boats and submarines. Thanks to prohibition, Brown notes, "the profit incentives to find ways over, under, around, or through any border infrastructure are high, and the cartels have more than enough money to spend on R&D."

When Trump allows himself to contemplate the possibility that his wall will be less than 100 percent effective at stopping heroin, methamphetamine, cocaine, and marijuana from entering the country, he still thinks the barrier's boost to interdiction efforts will be wll worth the money spent to build it. The Department of Homeland Security estimates that Trump's wall will cost about $22 billion, and Senate Democrats say the tab will be closer to $70 billion. But "I think I'll do it for $10 billion or less," Trump told A.P. "If we stop 1 percent of the drugs because we have the wall…that's a tremendously good investment."

It's a mystery how Trump made that calculation, but it seems to depend on the assumption that stopping 1 percent of drug shipments means reducing the supply of drugs by 1 percent. That is not how interdiction works, to the extent that it works at all. Cartels replace the drugs lost to interdiction, and they can do so pretty cheaply, since drugs acquire the vast majority of their value after entering the country. Given the economics of prohibition, the most drug warriors can reasonably hope to accomplish by intercepting shipments is forcing drug dealers to charge their customers more by raising production and distribution costs. But because the replacement cost is low, interdiction is unlikely to have a significant and lasting impact on retail prices.

"With few exceptions and despite increasing investments in enforcement-based supply reduction efforts aimed at disrupting global drug supply," a 2013 study published by BMJ Open concluded, "illegal drug prices have generally decreased while drug purity has generally increased since 1990. These findings suggest that expanding efforts at controlling the global illegal drug market through law enforcement are failing." There is no reason to expect that seizing 1 percent of incoming drugs would have a noticeable effect on consumption, let alone enough to justify the cost of Trump's wall.

Trump either does not understand any of this or assumes that voters don't. "I'm going to create borders," he promised in a campaign video. "No drugs are coming in. We're gonna build a wall. You know what I'm talking about. You have confidence in me. Believe me, I will solve the problem." While Trump's promises may sound especially grandiose and childlike, any politician who pushes supply-side solutions to drug abuse is engaged in the same basic scam.