CURIOUSER and curiouser.

The referendum on Scottish independence could hardly be simpler, especially if, as now seems likely, there is a single question. The constitutional future of Scotland will turn on voters making a mark in one of two small boxes. Yes or no? Yet the ramifications of a yes vote are hugely complex, as evidenced by the argument around the issue of retaining sterling as the currency, when the government of an independent Scotland would have no input into monetary policy.

Now we must brace ourselves for a yet more arcane argument about citizenship and the franchise. Professor Jo Shaw of Edinburgh University's School of Law, an expert in the intricacies of citizenship in Europe, has told the Scottish Affairs select committee that the independence debate must encompass consideration of the precise definition of citizenship and how different political factions might seek to manipulate it to their advantage.

There is a choice to be made. Currently, Scottish Parliament elections employ the same roll used for local elections, so the franchise is based on residency and any Scottish-based EU citizen can vote. However, in Westminster elections the franchise is based on a roll that is restricted to Commonwealth citizens and includes British ex-pats for 15 years after leaving these shores. There is a logic to this asymmetry when the regional legislature determines local issues affecting all residents, such as health and education. But in an independent Scotland would it be right for a temporary resident from Poland or Portugal to have a say in deciding future defence or foreign policy, while someone born and bred here but living elsewhere would be excluded? There was a similar debate about the franchise for the referendum itself.

The independence debate is by its nature very complicated and further probing turns up further questions. For instance, Prof Shaw invites us to consider a situation where many thousands of people living in Scotland were to opt to retain their British citizenship following independence. Like other British ex-pats, they would be entitled to continue voting in Westminster elections, provided they had lived elsewhere in the UK in the last 15 years. In some European countries, governments have created entire constituencies specifically for ex-patriates. This opens the possibility of perhaps hundreds of thousands of Scottish residents continuing to vote in Westminster elections, as well as presumably using their Scottish residency to vote for the government in Edinburgh. Is that fair?

If a post-independence Scottish government retained residency as the basis for the franchise, it would become the first EU member to do so. If it reverted to a narrower definition of citizenship, some of those who can vote in the referendum, would be denied participation in future elections.

And what if, having achieved independence, the SNP fractures between right and left? As in countries such as Hungary and Slovenia, could a right-wing conservative traditionalist government seek to manipulate the franchise, redefining citizenship to stress ethnicity and consanguinity? Could that be an unintended consequence of new statehood for Scotland? There needs to be some consideration of the different options on offer. The future of Scotland could hang on how we answer the deceptively simple question: What is a Scot?