Andrew Gelman is right that potential Scottish independence comes at a cost of losing influence in the larger United Kingdom (UK). However, the political calculus is not quite as Gelman describes. Look at it this way. The UK has lost some influence in European political structures. First, the European Parliament (EC) is essentially a do-nothing body that nobody cares for. So losing influence there is no big deal. However, the British have also lost influence in the European Commission, where it is represented by an appointee of HM Government. And that’s a loss.
The counter balance to this is not the gain in self-rule, as Gelman describes, but rather leaving the sphere of influence of the EC. As it will no longer be a member of the European Union (EU), it will no longer be subject to its rules and regulations. (This sets aside the fact the EU will exact punishing tribute on the British in order to get a trade deal, and, frankly, the British will likely end up more integrated with Europe than it is now, but that’s a detail of the current political structure.)
In Gelman’s calculation, if Scotland leaves the UK, Texas leaves the United States, or Québec leaves Canada, the downside is that the UK, US, and/or Canada will move in the opposite direction political as a sort of “conservation of momentum” and the cost is that each unit will lose influence over the whole. I agree with that, but the real counter balance is the lost influence of the whole over the subunit, which is what the radical secessionists in each of these jurisdictions want.
Image of the Battle of Stirling Bridge, unknown artist, from Hanley / Wikimedia Commons.