President Trump has repeatedly cited drug smuggling as a reason to build a wall along the Southern border. But my new Cato policy analysis shows that, if stopping drug smuggling is the goal, a border wall is about the worst possible investment. Here are a few of the main findings:

• Hundreds of miles of border fences built from 2003 to 2009 had no effect on marijuana smuggling.
• Marijuana legalization starting in 2014 has cut marijuana smuggling between ports of entry (i.e. where a wall would go) 78 percent from 114 pounds per agent in 2013 to just 25 pounds per agent in 2018. 
• Since marijuana is the primary drug smuggled between ports of entry, the total value of all drugs seized by the average Border Patrol agent fell 70 percent from 2013 to 2018.
• As a result, the average inspector at ports of entry made drug seizures that were three times more valuable than those made by Border Patrol in 2018. In 2013—prior to legalization—the average Border Patrol agent made more valuable seizures.
• By weight, the average port inspector seized 8 times more cocaine, 17 times more fentanyl, 23 times more methamphetamine, and 36 times more heroin than the average Border Patrol agent seized at the physical border in early 2018.

The best proxy measure for changes in drug smuggling is the amount of drugs seized by Border Patrol. To control for enforcement levels, the paper looks at seizures per agent. From 2003 to 2013, the rate of seizures per agent was virtually constant, even as the number of agents doubled and the government constructed hundreds of miles of border fences. But enforcement didn’t stop the flow—only when states, starting in 2014, started to fully legalize marijuana did marijuana smuggling decline (Figure 1).

Figure 1: Marijuana seizures and length of border fences

Because marijuana is primarily smuggled between ports of entry, the most valuable drug smuggling now occurs at ports of entry. As Figure 2 shows, Border Patrol was seizing more drugs—by value—in 2013, but by 2018, value of drugs seized at ports of entry was three times the value of drugs seized between ports of entry by Border Patrol. In other words, a border wall would not target the most valuable drugs crossing the border.

Figure 2: Value of drug seizures per agent by location of seizure

All of this should inform policymakers on how to address illegal immigration as well. Policies that make legal immigration easier undermine the incentivizes for black market activity. As Figure 3 shows, when lesser-skilled guest worker admissions increase, the number of apprehensions per Border Patrol agent declines. Since 1949, a 10 percent increase in guest workers was associated with an 8.8 percent decrease in apprehensions per agent. The current H‑2A and H‑2B guest worker programs for seasonal workers are already helping the situation greatly, but they could be improved and expanded on to cut illegal entries further.

Figure 3: Lesser-skilled guest worker admissions and apprehensions per Border Patrol agent

If Congress wants to address drug smuggling, it should legalize marijuana nationwide and invest in better ports of entry, not build a wall. The same lessons that drug smuggling have provided should apply to illegal immigration, cutting demand for illegal immigration by making more legal options available to immigrants.