Much of the debate about the future of the oil industry has focused on the fiscal impact of oil revenues generated directly by North Sea activity.

It has been argued that, because of the methodology used to produce demographic projections, the social and economic impact of this economic trend is not incorporated into the demographic assumptions of the Scottish Government's model. Consequently, there will be a greater than anticipated shortfall in revenues to meet future spending.

The Office for National Statistics (ONS) offers three possible future scenarios for the population of Scotland: a higher scenario, a principal scenario and a lower one. Using the higher scenario, the fiscal situation would be marginally better in 50 years' time than under the principal assumption. Under the lower scenario, Scotland's public finances deteriorate dramatically. All three ONS scenarios assume the same fertility and life expectancy but they make different assumptions about in-migration. In making their forecasts about the future of Scottish tax income, the SNP chose to work with the higher scenario.

Not surprisingly, this is the best case for supporting the Yes argument. It proposes plus-24,000 net in-migrants from 2018/19 onwards, rather than principal or lower scenario projections of plus-15,500 or plus-7,000. The ONS uses International Passenger Survey and NHS baseline data for its 2010 and 2012 migration projections.

It makes no adjustment for the decline of North Sea oil in the future. The implications can be seen in the projections for the areas that have benefitted from the growth of the oil industry in Scotland but which we might expect to be adversely impacted by a decline in the future.

In its 2010 projections, the ONS projected that the future populations of Aberdeen and Aberdeenshire will grow at more than twice the rate of Scotland as a whole and the working-age populations of these areas at three times the Scottish average. Based on past experience of changes in age and gender groups and taking account of fertility, birth and death rates, these projections are a logical outcome of the professional demographer's endeavour. Given the forecasts for the future of North Sea oil, however, the validity of these figures as the basis of fiscal forecasts must be questioned.

From the 1970s to the 1990s, net out-migration from Scotland was much higher than the natural increase and it is only in the past 12 years that more people have been consistently coming into Scotland than leaving.

Between 1971 and 1981, Scotland lost 194,000 people, an average of 19,400 a year. Scotland did not recover its 1971 population level until after the millennium. Since 2001, Scotland has experienced the fastest growth rates in a century. To base future estimates of Scotland's population on the trends in the last decade is, frankly, unwise.

The ONS made adjustments to its projections after the 2011 census data became available. The census showed it had substantially over-estimated net migration in the two years before the census. This led to the net figure for the principal scenario being reduced from plus-17,500 to plus-15,500.

The ONS, however, readjusted its projections "to gradually converge to the long-term assumption". It readjusted the figures up to approach its original assumptions about long-term net migration, despite recent evidence of its decline. This reinforces the suggestion that even the principal and lower migration assumptions of the ONS are likely to be significantly over-optimistic, never mind the higher projections used by the Scottish Government.

The Scottish Government argues that, with more liberal immigration policies, in-migration will be higher than in the rest of the UK.

However, a more liberal immigration policy will not work in the way intended unless there are increasing private sector employment opportunities. Without jobs growth that is greater than the natural increase in the working-age population, increasing in-migration will result in a growth in social tension and the divisive politics of xenophobia. The Scottish reputation for generosity of spirit will be sorely tested.