DONALD TRUMP’S signature campaign promise was to build a “big beautiful wall” along America’s southern border to keep out undocumented immigrants. The chief source of funding, he reckons, will be a one-time payment of $5bn-10bn from Mexico. A campaign memo released in April 2016 says the Trump administration will force Mexico to stump up by threatening to block money transfers from undocumented Mexicans living in the United States. This would be “an easy decision for Mexico” according to the memo. Indeed, Mexico received nearly $25bn in remittances in 2015 according to its central bank, equivalent to 2.3% of the country’s GDP. Over 98% of these remittance payments came from the United States.

But there are serious cracks in Mr Trump’s plan. First, blocking certain remittance payments requires the Patriot Act to be rewritten, which needs legislative action by Congress and would probably be challenged in court. Many experts also point out that such a rule would introduce new costs for all non-citizens transferring funds abroad, not just those sending money to Mexico. Finally, a new rule blocking money transfers from illegal immigrants would likely push these transactions underground, creating a black market for remittances and making such payments more difficult for government authorities to track.

However, some are not taking any chances. In November remittances to Mexico totalled nearly $2.4bn, a 25% jump over the previous year. Total remittances for 2016 are expected to reach $27bn, $2bn more than in 2015. And America’s southern neighbours may be pre-empting the Mr Trump administration in other ways. Since Mr Trump’s victory, flows of illegal migrants from Central America have increased. In November the US Border Patrol apprehended 47,214 migrants along America’s southwest border, an increase of 44% over 2015 figures. Faced with such trends, calls to “build that wall” may grow even louder.