What would make someone choose between British and Irish nationality? There is a consensus that rows over the Irish border have brought a referendum on unification much closer. But there is almost no discussion of what issues might determine how people in Northern Ireland vote in such a referendum.

You might object that “issues” are beside the point. Nationality, you might argue, is not determined by tax rates or education policy but by history and culture, poetry and memory, blood and speech. The reason that support for a united Ireland has grown north of the border over the past century, you might go on, is not that the Irish Republic has been outperforming the United Kingdom, but that the birthrate among Catholics, who generally identify as Irish, has been higher than among Protestants, who generally identify as British. As that bombastic loyalist clergyman, Ian Paisley, once put it: “The secret weapon of the Roman Catholic is the perambulator,” i.e., the baby carriage.