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.

But that is not the whole picture. If people voted purely along confessional lines, there might very well already be a majority for a 32-county Irish republic. In 1921, when Northern Ireland was founded, Protestants outnumbered Catholics 2-1. The last published census data, from 2011, showed the two populations almost the same size. The 2021 census, which will be published next year, may show the first Catholic majority.

Despite that demographic shift, opinion polls continue to back the status quo. The most recent major poll of Northern Irish voters, carried out in October, showed support for U.K. membership at 59%, versus 30% for a united Ireland. A significant minority of Ulster Catholics would vote to stay in the United Kingdom. Some of their reasons are practical: recognition that Britain’s vaccination program was better than Ireland’s, fears about jobs or pensions, doubts about whether Dublin would subsidize the province to the same degree that London does. But these things would count for nothing if they did not also feel that they could freely express their Irish identity within the U.K.

And it is here, it seems to me, that unionism (the wish to remain British) has an unremarked edge over nationalism (the desire to join Ireland). Whereas unionism assumes that you can be both British and Irish — indeed, unionism might be said to have begun as the idea that to be Irish was also to be British — there is no version of Irish nationalism that is not based on a rejection of Britishness.

This matters because Northern Ireland cannot be incorporated into either of its neighboring states without disappointing a big chunk of its population. The British government has always recognized that challenge. Hence its support for power-sharing, its readiness to let people in Northern Ireland take Irish passports and proclaim Irish nationality, and its willingness to let Irish nationals within Great Britain vote, claim social security, and exercise other rights as full citizens. Because so many British (including this author) have Irish family links, Ireland will never be seen simply as a foreign state in the way that, say, Finland is.

Oddly, though, the reverse does not apply. Every time Ireland has had to choose, it has emphasized its distance from Britain rather than trying to appeal to Northern Unionists. It was Eamon de Valera’s provisional government that insisted on putting up a border, complete with customs posts, precisely a hundred years ago, much to the regret of London, the preference of which, once the break became inevitable, was for an all-Ireland state loosely associated with Great Britain.

Had successive Irish governments wanted to woo the million-odd people on the island who identified as British, they might not have cut their remaining symbolic links with Britain in 1937, or remained neutral in the Second World War, or walked out of the Commonwealth in 1949, or promoted Irish as the national language.

Each time, they chose to diverge rather than to accommodate the people they aspired to rule. In doing so, they may well have been accurately reflecting their own voters’ wishes. A poll in the Republic last month showed that, while people would vote for a united Ireland by 60% to 25%, it was a very different picture if such unity was conditional on higher taxes (41% in favor, 43% against), altering the flag (27% in favor, 59% against), or rejoining the Commonwealth (23% in favor, 58% against).

All these are legitimate choices. Ireland is under no obligation to seek to accommodate the British identity of the population it aspires to incorporate. But it should not surprise anyone when that population prefers to stay as it is, which is why, a hundred years on, Northern Ireland isn’t going anywhere.