The big hole in Trump’s wall

By W.Z.

FEW of Donald Trump’s campaign promises are as memorable as his pledge to build a “big, beautiful wall” along America’s southern border. But while the planned barrier may play well with Mr Trump's base, it addresses a problem that has largely abated. Illegal immigration has been declining since 2007. A new study by economists Gordon Hanson, Chen Liu and Craig McIntosh of the University of California, San Diego argues that it will fall further still.

During the economic boom of the 1990s and early 2000s, unauthorised immigrants, mainly from Mexico, flocked to America. The total number of undocumented immigrants rose from 3.5m in 1990 to a peak of 12.2m in 2007, but came to a halt after the financial crisis, according to The Pew Research Centre, a think-tank. Part of this decline has been a result of policy: the Obama administration made it a priority to stem the flow of immigrants, while also shielding long-term residents. But economic and demographic trends have also played a part.

Flows of legal immigrants are largely fixed because of the visa system. In contrast, flows of illegal immigrants are more dependent on economic considerations. During the financial crisis in American, as wage growth slowed, Mexican workers with lower education levels had less reason to move north. Given the long-term decline in wages for low-skilled workers in America, this trend is not likely to reverse.

Demographic changes have also made a difference. While America's baby boom ended in the early 1960s, Latin America’s birth rate fell more gradually. This mismatch meant that there was a large stock of young Latin American workers who could drastically improve their earnings by emigrating. But birth rates in Latin America have since almost converged with those in the United States (see chart). Using a model that incorporates both demographic and economic factors, Mr Hansen and his colleagues predict that migration from Latin America will fall sharply over the next two decades, wall or no wall.

The Obama administration's policy, meanwhile, has had two effects on the demographic profile of America's illegal immigrants. First, fewer young migrants are entering the country. Second, those who haven't been deported are staying on for longer: the share of undocumented immigrants who have lived in America for at least ten years has risen from 41% in 2005 to 66% in 2014. Recent work by George Borjas of Harvard University shows that while wages have historically been lower for illegal immigrants than legal immigrants or native-born citizens, they are rapidly rising. Republicans are reluctant to go after this group; Mr Trump has at times said that he wants the “good ones” to have a have path to legal residency.

America has been fortunate to have a supply of young migrants to bolster its workforce, but this advantage is fading fast. Five million Mexican-born immigrants aged between 15 and 40 years old currently live in the United States, but that number is expected to plummet in the coming decades. Mr Hanson notes that the population of undocumented immigrants who stay in America is ageing, and because they are not covered by Medicare or Medicaid, they will eventually be faced with the choice of either paying high medical bills or returning to their native countries. The main problem with illegal immigrants in America is not how to prevent new arrivals but rather what to do with the 11.1m or so who remain in the country in limbo. Tackling this problem will be much harder than building a wall.