A generation of political consultants was raised on the irritating slogan "It's the economy, stupid". But immigration now rivals economics as the driving force in Western politics.

In economic terms, it makes sense for rich, ageing countries like Britain and the United States to draw in new workers and citizens from poorer, younger neighbouring countries. But the election of Mr Donald Trump and the vote for Brexit were both fuelled by fears of uncontrolled migration, with Mr Trump promising to "build the wall" along the Mexican border, and the Brexiters vowing to "take back control" of borders from the European Union (EU).