#4 Mean Reversion
This mechanism's perhaps a bit unfair since, if it's true, then it applies to both independence and union. If it's not true though, then conclusion must be that we expect gains from independence. In either case, the economic outlook is brighter than often painted.
In A conversation on the economics of independence, I suggested that there must be an institutional reason for the regions of the UK excluding London and its environs occupying all of the poorest places in the ranking of northern European regions which had never been communist. This contention was made because I could think of no structural or fundamental reason for these UK regions to be so disadvantaged. If this contention is correct then we should expect institutional change, such as independence, to be accompanied by economic gains.
However, there is another possibility: luck. Perhaps the regions of the UK excluding London and its environs have experienced an unfortunate series of bad outcomes. If this is the case, then we should not expect this run of bad luck to continue, and these UK regions should "revert to the mean".
Certainly Scotland's experience over the past century has been particularly poor in a European context. This can be most clearly seen from population statistics. As I discuss in Population Records, "whether economic success determines relative demographics or vice versa, these statistics clearly co-vary and looking at the outcome of relative migration can tell us about the outcome of relative economic performance. Over the 20th century, Scotland did terribly on net migration and population growth."
If this was bad luck, then we should expect to do better over 21st century because of simple mean reversion. If this was due to institutional features, then we should expect to do better by changing institutions.