

Republican presidential nominee Donald Trump speaks during the second presidential debate at Washington University in St. Louis, Sunday, Oct. 9, 2016. (Saul Loeb/Pool via AP)

We’re also letting drugs pour through our southern border at a record clip. At a record clip. And it shouldn't be allowed to happen. -- Donald Trump, at the second presidential debate.

In a debate performance characterized by personal insults, outlandish claims and a threat to jail his opponent in the event that he won the presidency, it was easy to overlook Donald Trump's claim, above, about the flow of drugs into the U.S.

Broadly speaking, Trump's assertion that "drugs pour through our southern border at a record clip" is not supported by the available data.

Drug seizures at the southwest border are the best barometer for measuring the amount of drugs flowing into the country from Mexico and points south. Data from U.S. Customs and Border Protection shows that the total amount of drugs seized at the Southwest border (along California, Arizona, New Mexico and Texas) has dropped precipitously over the past five years, from roughly 2.5 million pounds of drugs in 2011 to about 1.5 million pounds in 2015.

Nearly all of this decline is due to the drop in marijuana seizures over the same period. As a plant, marijuana is by definition a much bulkier drug than refined and concentrated substances like cocaine, heroin and methamphetamine, which make up the remainder of drug seizures. Marijuana is also much more widely used than other illicit drugs, so its lion's share of seizures by weight comes as no surprise.

There's more variation in the numbers when you look at other drugs. Cocaine seizures fell by more than half, from 8,763 pounds in 2011 to 4,294 pounds in 2015. Heroin has actually risen, from 387 pounds in 2011 to 515 pounds in 2015. But heroin seizures were even higher in 2013 (559) and 2014 (575), so the idea that heroin is coming over the border at a "record clip," as Donald Trump says, does not appear to be true.

The only drug for which the CBP numbers support Trump's claim is meth -- customs officers seized 6,429 pounds of methamphetamines in 2015, up more than threefold since 2011 and higher than the number in every year in between.

Trump's argument about drugs coming over the border is part of his larger push for tighter border controls to stop undocumented immigrants from entering the United States. The centerpiece of that argument is a plan to build a wall along the entire Mexican border and make Mexico pay for it.

From a financial, legal and logical standpoint, many experts have criticized that plan as unworkable. On the merits, it's also highly unlikely a wall would keep drugs out either.

Traffickers don't only smuggle drugs overland, after all. They dig tunnels underneath the borders. They send drugs via submarine. They sneak a significant amount of contraband into the country via airports.

As long as there's demand for cocaine or heroin or meth, in other words, traffickers have demonstrated that they'll find a way to bring drugs into the country regardless of how tall any border wall might be.

The failures of costly supply-focused measures -- like shutting down drug dealers and burning down drug crops -- was one of the toughest policy lessons of the drug war. Ironically, the one thing that's had the greatest impact in decreasing the flow of drugs across the border -- particularly marijuana -- has been the loosening of restrictions on drug use here in the U.S.

"If the U.S. continues to legalize pot, they'll run us into the ground," a Mexican marijuana grower told NPR in 2014, at the end of the first year of Colorado's recreational marijuana markets.