This side of paradise. (Sam Hodgson/Bloomberg News)

When asked during his presidential campaign how he would stop America’s raging opioid overdose epidemic, Donald Trump said “cut off the source, build a wall.” No one disputes that many drugs enter the United States via the Mexican border. But does this imply that trying to seal off the border would reduce the nation’s drug problem?

The influence of a public policy change, for example tightening or loosening border security, varies depending on the baseline from which one is starting. Compared to having no border security, the United States' current, moderate level of drug law interdiction at its borders clearly has an impact. University of Maryland Professor Peter Reuter points out that “it costs about $10,000 to move a kilogram of cocaine from Bogota to New York City, but you can ship the same amount of legal product with Fedex for $150.” The costs that drug traffickers endure to evade patrols, bribe officials, replace confiscated drugs, build tunnels and the like are passed on to consumers, significantly raising drug prices and thereby discouraging use.

But moving further along the curve of enforcement intensity, would a massive ramp-up of border control (e.g., by building a wall on the Rio Grande) beyond its current moderate level push illicit drug prices to unprecedented heights, thereby dramatically curtailing use?

“Economists, even lapsed ones like me, assume as a matter of doctrine that tougher enforcement should raise prices, but drug markets have just not behaved the way we expected” says Reuter. Indeed, drug policy research across a wide range of areas, including augmented border controls, arrests and incarceration, suggests that the returns of tougher policies diminish rapidly.

Intensified border enforcement can also cause negative spillovers into other areas. In 1969, the U.S. government launched Operation Intercept, flooding the Mexican border with agents and military hardware dedicated to stopping the marijuana trade. The length of lines at border crossings grew enormously in response to the more laborious security procedures, harming tourism and other business interests on both sides of the border. Innocent travelers resented the intrusion on their civil liberties caused by their cars and bodies being searched. Within three weeks, the binational outcry prompted the government to abandon the operation.

Even if the Trump administration did not learn from history and were willing to endure the substantial costs of a border blockade in exchange for the very modest reduction in heroin imports that it might produce, the opioid addiction epidemic would still be with us. Over 200 million opioid prescriptions are written in the United States each year, and deaths from prescription opioids well outpace those caused by heroin. Although it may appeal to nativist sentiment to blame the U.S. opioid epidemic on Mexico, it’s mainly a homegrown phenomenon.

Keith Humphreys is a professor of psychiatry and mental health policy director at Stanford University.