Apart from death and taxes, one of the most certain things in life is a prediction from a southern writer that Catholics will soon outnumber Protestants in Northern Ireland, leading quickly to a majority for Irish unity.

The front-runner in these attempts to disconcert unionists used to be the southern historian and writer Tim Pat Coogan, a former editor of Eamon De Valera's Irish Press newspaper.

The baton was later taken up by David McKittrick, Belfast correspondent of the liberal London Independent.

The most recent entrant into this field is the Dublin economist, broadcaster and journalist David McWilliams.

McWilliams came to prominence for correctly describing the Irish economic boom of the early-Noughties as a credit bubble that would soon collapse. His celebrity now depends as much on his ability as a stand-up comedian and co-founder of the Kilkenomics festival.

In true Irish fashion, McWilliams has managed to make economics amusing.

Although not in the league of Michael Lewis, or P J O'Rourke, he has nonetheless built something of an international reputation. His connection with Northern Ireland is through marriage into an east Belfast Protestant family, a link he makes good use of in his writing and talks.

He was the star turn at an Ulster University Brexit symposium last year, in which he contrasted the economic failure of the north with the glittering success of the Republic. He particularly stressed the replacement of Ulster as the dominant seat of manufacturing in Ireland with that of the modern Republic.

This was done in a light-hearted vein, but he did not always remain on the right side of the narrow line between comedy, parody and triumphalism. This was particularly the case with his predictions of eventual Irish unity, which appeared to be based on thin evidence.

All of this might have been of only local interest until McWilliams was recently given front-page coverage in the Financial Times, an influential paper with a wide international readership. Again, the same themes were the failure of the northern economy and the inevitability of Irish unity.

The core of his case on unity was that the Catholic share of Northern Ireland's population had been rising for almost a century and would soon overtake that of Protestants.

In doing so, he committed the simplest of statistical errors - drawing a straight line between two points in time and assuming that the trend would continue unchanged in future.

He used the 2011 census to argue that Catholics were already a majority among the under-four-year-olds and implied this would continue to build a majority as these and future cohorts grew up.

In fact, the percentage of four-year-olds described (presumably by their parents) as Catholic was 44% of the total population of under-fours. What McWilliams had done was to compare the number of Catholic under-fours with those described in the census as Protestants.

In doing so, he left out those who had no religion and the substantial number of those whose parents had refused to answer the religion question.

The 2011 census clearly shows that the percentage of the population who described themselves as Catholic had peaked among those born almost two decades ago and has subsequently slowly declined. Among those who were teenagers in 2011, 46% said they were Catholics, but among the under-fours in 2011 the figure was lower, at 44%.

If this falling trend continues, there will never be a Catholic majority. Since Catholic birth-rates are now close to those of Protestants, it seems likely the trend will, indeed, continue.

Predictions for the future share of Catholics in the population also depend on migration and here there is major new factor. One-in-20 of Northern Ireland's Catholics is now from Poland, Lithuania, Portugal and the Philippines. The future constitutional preference of these immigrants and their children is hard to predict. It is certainly wrong to accept McWilliams' glib equation of Catholicism with Irish nationalism. The 2011 census showed that only half of Northern Ireland's Catholics identify as "Irish" and under half have an Irish passport. The authoritative Northern Ireland Life and Times Survey shows that, even in 2017/18, the proportion of Catholics who support eventual Irish unity is 41%. Only 7% express a desire for immediate unity. Among the whole population of Northern Ireland, only 20% supported Irish unity in the 2017/18 survey.

McWillams had assumed that Brexit had increased support for Irish unity as a means of remaining in the EU. In this, he echoed the assumption of Sinn Fein that because 56% voted Remain in the 2016 referendum, this must have included many Protestants.

However, the assumption that this may have implied growing support for Irish unity among Protestants appears to be wrong. The NI Life and Times Survey shows virtually no change in support for Irish unity among self-described Protestants since the pre-referendum years.

Moreover, the 2017 General Election showed a huge 108,000 increase in the DUP vote and a one-third collapse in the vote of the UUP, which then supported Remain, but has subsequently changed its stance.

It seems most likely that, while some Protestants preferred the UK to remain within the EU, this did not mean that they preferred Northern Ireland to remain in the EU if the UK was out.

More movement in support for Irish unity is observed among Catholics, or those with no religion, but even here the change is not large.

We do not know why so many Catholics support Northern Ireland's place within the UK, but we can guess that the NHS, free school education, a generous social security system and low-cost housing are all factors.

McWilliams tries to argue that the Republic of Ireland is a much more successful economy and that its people are richer. He seems not to know that Professor John Fitzgerald of ESRI in Dublin (and a more eminent economist than himself) has recently calculated that living standards are 25% higher in Northern Ireland compared to the south.

Although wages are generally higher in the south, higher taxes and fees and inferior levels of public-service provision mean that northerners do better, even before we take cheaper housing into account.

Nor does the north's dependence on public spending indicate a weak private sector, as McWilliams simplistically suggests. Rather, it reflects the way the UK works.

The high birth-rates of the past created an excess of labour in Northern Ireland, which a sedately growing UK economic union was not designed to accommodate. Instead, public services (and hence jobs) were provided for a growing population.

While the south relies on its rip-off tax-haven status to employ a growing population, the UK provides public spending in a way that is fully sustainable. Many seem to like it that way.

Dr Graham Gudgin is chief economic adviser of Policy Exchange, described as the most influential think-tank on the centre-Right. He is honorary research associate at the Centre for Business Research at the University of Cambridge and visiting professor at Ulster University.

Belfast Telegraph