A leading academic has suggested that the 2021 census in Northern Ireland could show Catholics outnumbering Protestants for the first time. Dr Paul Nolan, an expert in social trends and the peace process, told the BBC that the figures of 48 per cent Protestant to 45 per cent Catholic reported in the 2011 census were changing rapidly, with a younger Catholic population making up a larger percentage of the workforce and a majority of the school-age population.

This demographic shift is highly sensitive in Northern Ireland since the political system is based on the sectarian divide and the principle of whether the region should remain in the UK or be united with the rest of Ireland. Protestants overwhelmingly vote for the unionist parties, the DUP and UUP, Catholics vote for the nationalist Sinn Féin or SDLP. The small centrist Alliance and Green parties win support from those who don’t identify with either camp. So even micro-trends such as the exodus of Protestants from rural border areas, or the sectarian makeup of newly built housing estates, are closely monitored to see which side is gaining and which is losing.

Unionists already had a shock in last year’s Assembly election, when Sinn Féin won 27 seats to the DUP’s 28, and was little more than a thousand votes behind the DUP, with unionists failing for the first time to secure a majority of Assembly seats. The gap between the two camps has been narrowing for a long time, but many unionists were still not psychologically prepared for a situation where they didn’t have a secure majority. The approaching census will make them even more jittery.

Beyond the headline figures is a more complicated picture. The census measures religious identification rather than religious observance, which is significantly higher than in mainland Britain but still in decline. There is also a stubborn minority that identifies with neither major tradition, and either ticks “no religion” or leaves that section blank on the census form.

There is an old joke about an atheist in Northern Ireland being asked whether he is a Protestant or a Catholic atheist, but it is true that diversity monitoring forms on job applications ask about “community background” rather than religion. Since religion is a tribal marker, anti-discrimination law is as much concerned with perceived religion as actual practice.

As a result, Northern Ireland’s statistical agency Nisra has developed a range of census questions that can be used to interpret the results. In the 2011 census, the religion question showed just less than 41 per cent declaring themselves Catholic, 36 per cent for the three main Protestant churches (Presbyterian, Anglican and Methodist) and around 6 per cent for other denominations (mostly small Evangelical or fundamentalist groups). The figure of 48 per cent Protestant to 45 per cent Catholic came from the “religion brought up in” question.

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