Reading about social choice (the theory about voting rules and in general how to go from individual preferences to collective choices) teaches several lessons about the dangers of many secessionist referendums, like the Brexit referendum or the aspiration of Catalan and other nationalists in rich European regions to secede from their states. Social choice does not present many problems when there are only two options. There is not much scope for strategic voting, there is little risk of indeterminacy and irrelevant alternatives do not have any influence (there are none at that stage), and there is no difference between plurality and majority. But the manipulation comes in the reduction of complex phenomena to only two options. Once there are only two options, the battle for framing is over. The solution is not simply to increase a number of apparently simple options to three or more, because then the question keeps giving the false impression of simplicity (it would have been hard to know precisely what devolution max meant in Scotland). And then the usual procedure of plurality voting may deliver a winner that is hated by a majority (as opposed to the Borda count, which however is vulnerable to comparisons being dependent of irrelevant alternatives), and to strategic voting. The best option for sovereignty issues in advanced democracies is to reach a broad agreement for a yes/no question on a detailed proposal (like the Irish referendum on the Good Friday agreement). That is the appropriate framing in a democracy that wants to preserve tolerance and reasoned debate, leveraging the best practices in representative and deliberative democracy. A second lesson is precisely about the need for reasoned and informed discussion, something that was dramatically absent in divisive plebiscites such as the Quebec referenda of 1985 and 1990, or the Scottish referendum of 2014 or the Brexit one last year. Finally, as Amartya Sen reminds us in the final pages of the expanded edition of his book on Social Choice, the view of outsiders should be welcomed in any debate to avoid the excesses of parochialism. It is very difficult to know about social choice and agree with secessionist referendums or at least not to have some serious doubts. For the main conclusions of social choice are i) that the will of the people is ill defined and ii) that there are many possible voting rules in democracy, none of them being perfect.