Policy drives evidence in modern political debate. Nowhere is this more true than in discussion of the economics of immigration to Britain.

In 2007 the home secretary lauded “the purity of the macroeconomic case for migration”, and the immigration ministerLIAM BYRNE claimed: “There are obviously enormous benefits of immigration.” He even quantified the benefit at ￡6bn a year (though this is hardly “enormous” relative to gross domestic product of ￡1.5tn). Official predictions grossly underestimated the numbers who would come when the border was opened to Poles in 2004.

By 2012, however, the home secretary was claiming that, without immigration, house prices would be lower and wages higher, and that every 100 immigrants put 23 British workers out of a job. Official predictions appear to have grossly overestimated the numbers who would come in 2014 when the borders were opened to Bulgarians and Romanians.