THIRTY feet high, built of concrete and steel, eight prototypes for Donald Trump’s wall stand in the dusty ground near the Otay Mesa border crossing in San Diego. Later this month a private company will start testing the slabs to see how they withstand attempts to climb over or tunnel beneath them.

On October 26th, when Donald Trump declared America’s opioid crisis a public health emergency, he said that his signature infrastructure project was a big part of the solution. “An astonishing 90% of the heroin in America comes from south of the border, where we will be building a wall which will greatly help in this problem”, he said. “It will have a great impact.” He had previously suggested that the wall should be see-through to prevent injuries from smugglers launching 60-pound bags of drugs over it (two of the prototypes are transparent). Mr Trump has long argued that a border wall would reduce undocumented immigration including that of drug dealers. But the available evidence suggests that a wall would have no effect on heroin supplies and could even increase the number of undocumented workers in America.

Nearly half-a-century of the war on drugs has demonstrated that the most effective approaches target demand for illegal narcotics rather than supply. During that time there has been far tougher sentencing for drug criminals and a considerable expansion of the Drug Enforcement Agency, Border Patrol and other policing efforts; alongside military involvement on the border, at sea and in drug producing countries. But in 2014, John Kelly, then head of the United States Southern Command, testified to the Senate that only 20% of the drugs coming from Colombia were intercepted. That the retail price of heroin fell in America from above $3,000 per gram in the early 1980s to $500 per gram in the last few years illustrates the failure of efforts to restrict supply.

Control efforts along the Mexican border appear to be particularly ineffective in dealing with heroin. Michelle Keck and Guadalupe Correa-Cabrera of the University of Texas looked at the number of hours border patrol agents spent patrolling the Mexican border, drug seizures and drug process. They found that more patrolling was associated with no change in the level of heroin seizures.

A wall, furthermore, would largely be an irrelevance to the trade. Most heroin that comes across the Mexican border is transported by vehicle and smuggled through official entry points because it is a low-volume, high-value commodity. The Criminal Justice Policy Foundation estimates that the entire quantity of pure heroin consumed in America each year would take up 633 cubic feet –or about one sixth of the cargo volume of a single eighteen-wheeler truck. By way of comparison, the Department of Transport reports that 327,647 loaded truck containers entered America from Mexico in July 2017 alone. And a 60-pound sack of heroin would be worth many millions of dollars, suggesting it is extremely unlikely that a smuggler would risk launching it over a wall to land on an unsuspecting border patrol officer on the other side.

Regarding the movement of people, Douglas Massey and Karen Pren of Princeton University along with Jorge Durand of the Centro de Investigación y Docencia Económicas studied the impact of border security on undocumented migration in a 2016 paper for the American Journal of Sociology. From 1986 to 2008 there was a fivefold increase in the number of border patrol officers. Over the same period, there was also a fourfold increase in America’s undocumented population. And the probability of apprehension at the border remains similar to levels see in the 1970s.

Mr Massey and colleagues found that the likelihood that a potential migrant would decide to take their first undocumented trip to America was predicted by economic factors on both sides of the border as well as the age of the potential migrant; Border Control funding and staffing had no effect. But increased enforcement did have an impact: it shifted undocumented migration routes from California and Texas towards the (more inhospitable) border with Arizona, which made crossings more costly and more dangerous. And that increased the incentive for undocumented workers to stay in America once they had arrived rather than move back and forth across the border. The probability that undocumented migrants returned to Mexico was strongly and inversely associated with border control spending.

Figures from the Pew Research Center illustrate the impact. Pew has estimated that in 2007 there were 12.2m unauthorised immigrants in America, up from 3.5m in 1990. This was the peak: by 2014, those numbers had dropped to 11.1m. But in the 1990s, about one third of the undocumented population had been in America for less than five years, one third for between five and ten years and one third for over ten years. By 2014, two thirds of the undocumented population had been in the country over ten years and only 14% for less than five years.

Tougher border enforcement does not greatly reduce the capacity to enter America for either heroin or people, but it does reduce migrants’ desire to go back home.