SINCE the control of a nation’s borders, its qualifications for citizenship and rights of residence are, like the power to issue currency or raise armed forces, almost axiomatic features of sovereignty, it’s no great surprise that the Scottish National Party would like Holyrood to control immigration.

True, the SNP policy on immigration differs from that advanced by most political parties, in that it wants more of it, rather than to reduce it. It deserves credit for this, because it’s not an especially popular stance, and there’s some evidence that it may cost it votes. But on demographic and economic grounds and, for the most part, general principles, the case for increasing immigration is overwhelming, because the arithmetic is pretty well indisputable.

By contrast, the argument that mass immigration is overwhelming the native population, or national characteristics, social norms or the like is nebulous, tricky to demonstrate and (especially for those who think of themselves as “progressive”) faintly disreputable, if not downright racist. Indeed, until just a few years ago, most political parties simply ignored or aggressively disputed such cultural anxieties, though examples of social breakdown in places such as Govanhill, or of schools where the majority of pupils do not have English as a first language, indicate that those concerns are far from illusory.

Nonetheless, such worries may be overstated, manipulated by populist politicians, or simply misdirected – with the failures being policy, or resource allocation, or misguided cultural relativism, rather than some essential feature of immigration. They’re at any rate not insurmountable; polling in January suggested that, since the Brexit vote, the UK electorate’s attitude towards migration has become more positive, with 45 per cent thinking its impact positive and only 31 per cent negative – almost exactly the reverse of the figures in June 2015.

This applies to Leave and Remain voters; perhaps because Leave voters (only a tiny minority of whom ever listed immigration as their chief concern) believe the Westminster Government will soon have control, or perhaps because their aim was always control itself rather than blanket opposition to immigration. The next Prime Minister will almost certainly ditch the blinkered (and, it turned out, unattainable) target figures that Theresa May so stupidly clung to, and voters seem perfectly relaxed about the prospect.

But that distinction between control and numbers is the reason why devolution of immigration, like freedom of movement within the EU, is not really an option while Scotland remains within the UK, or for that matter, within the EU.

The SNP is right to point to an impending demographic crisis in paying for pensions, welfare services and care for the sick and elderly, because the Scottish birthrate is at its lowest for 17 years, and the population is expected to grow by just 4.4 per cent by 2040, considerably less than comparable EU countries.

The Scottish Government’s also right that immigrants are a net economic boon; the gain in tax revenue and productivity is so much more than the costs that it’s evidentially impossible to dispute, even for those who argue that wage suppression in the lowest-paid jobs, or costs in education, health and housing are significant (which they’re not). Of course, Holyrood could try spending less, or attracting people from elsewhere in the UK with lower tax rates, but if it’s not prepared to do that, immigration is one solution to the black hole in the budget.

But you can’t realistically determine your own immigration policy while part of a larger union, whether it’s the EU or the UK, even if Scotland’s current concern is that Brexit may reduce the number of migrants. For now, anyone from any EU country can still set up in any other one (inward EU migration to the UK still exceeds those leaving) and anyone admitted to the UK can settle anywhere within it. There are forms of devolution that allow some incentives within that free movement; regional subsidies to encourage industries or housing, or to promote population growth (the Greek island of Antikythera, of ancient mechanism fame, now has a mechanism for paying you €500 to go and live there).

The opposite can be done, by restricting access to aspects of welfare or services, although to a limited degree within EU rules. The UK, under Tony Blair’s government, chose not to apply them, but Scotland does apply some restrictions: for example, the length of time you must be resident to qualify for university tuition – something which currently disqualifies only rUK applicants.

In fact, at the moment, Scottish Government spending priorities and taxation levels provide a disincentive for those of working age, particularly higher earners, and a strong incentive for students and retirees (who gain social care provision and other benefits unavailable in other parts of the UK).

I share Holyrood’s favourable view of immigration, and of its economic benefits – perhaps even its economic necessity – but it’s possible to provide more general incentives in areas that are already devolved. And it would be a good thing for the UK to rethink income qualifications and skills requirements; what’s regarded as suitable for south-east England doesn’t necessarily fit the Highlands and Islands.

To devolve immigration policy in its entirety, however, requires the erection of distinctions and controls between territories that differ; it is more or less a de facto declaration of full independence. A good reason, naturally, for the SNP to be keen on it, but a fairly obvious reason for it to remain a reserved power while the Union exists.