By Dr Philip Wood

Alex Salmond's plans for an independent Scotland involve a number of leaps into the unknown. One of these is the escalation of Scottish migration to a target of 24,000 per year, almost double current levels.

His policy is a reaction to Scotland's ageing population, which would give an independent Scotland a dependency ratio much higher than the rest of the UK. This poses serious questions over Scotland's ability to pay out pensions, a problem exacerbated by the uncertainty expressed in the markets over the prospect of independence, and by Salmond's own promises to keep the age of retirement at current levels.

The first minister's notion that migration can cure the problems of an ageing society has supporters among much of the European elite. However, as Paul Demeny has argued, it is very unclear that the kinds of migration required to offset this kind of demographic change would actually leave any European state "fairer, stronger, richer or (in the long-term) younger".

Firstly, and most importantly, the rejuvenating effects of migration will only be temporary: migrants too will age. There are benefits to reaping the labours of men and women educated abroad, but the host state will still have to pay their pensions.

Surveys from the United States, undertaken by the think-tank Migration Policy, suggest that the bottom ten per cent of the economy will experience the depression of wages from competition and that a further ten per cent fear themselves to be subject to wage depression. This figure is likely to be even higher in European societies where people are less able or willing to move for work.

Thirdly, this scale of migration is unlikely to be conducive to social cohesion in the long-term. This is most immediately true where immigration causes new competition for schools, housing and healthcare, the pillars of the Scottish welfare state that Salmond has repeatedly emphasised to voters. But it may also lay the seeds for problems in the future where migrants recruited for economic purposes continue to recruit their own relatives through systems of chain migration.

Such systems can long outlast the initial motives that prompted a state to recruit a first wave of migrants. This was the scenario for migrants from Pakistan in the 1960s, where men recruited to prop up declining industries in the north of England became the magnets for future waves of migration from their home regions. The long-term effects of such migration have not been positive, and rates of employment and salary levels for many of the South Asian groups in the UK that are involved in chain migration have remained poor.

Of course, one can certainly hope to cherry-pick the best migrants from the global market using a selective, points-based system, as Australia and Canada have done. But recruiting specific highly skilled and high-value migrants is a competition, where Scotland needs to be able to offer a better working environment than European and Commonwealth rivals.

Points-based migration regimes, including that currently used by the UK, have proved hard to administer in practice, not least because they require governments to anticipate the demands of the market. Research by Centre Forum has also pointed to the inability of points-based systems to recognise migrants' soft skills (especially language skills) and a heavy gender bias towards men. In Canada immigrants experienced an unemployment rate twice that of native workers, which suggests that its points-based system remains a work in progress.

There are great potential advantages to be drawn from skilled migrants, but it would be very unwise to rely on migration to fulfil the kind of economic promises that Salmond is making. His unrealistic hopes for the role of migration in the Scottish economy look like a triumph of hope over experience.

Dr Philip Wood is an associate professor at Aga Khan University

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