The recent reversal, under domestic and international pressure, of the decision in Iraq to regard the new constitution as ratified unless two-thirds of all registered voters reject that document, draws attention to supermajorities and voting rules in general. It's easy to see that with expected voter turnout of 40 or 50%, a required two-thirds no vote by registered voters gives voters almost no say at all. The drafters, or agenda setters, then have all the power, subject to the constraint that the constitution may need to be accepted on the street, as opposed to legally ratified.
But what is the right denominator in a supermajority vote?
My colleague, Adrian Vermeule, has been exploring this question with respect to legislative votes. And why supermajorites? The intuition with regard to a constitution is sound; among other things, it is that there may well be fundamental rules that achieve something close to consensus, and so drafters should be looking to achieve that consensus, rather than a mere simply majority. But in Iraq, and perhaps in more settings than we have recognized, the original supermajority requirement (two-thirds of those who vote) may be too much. And whatever the rule of ratification, the drafters have enormous power. The voters are threatened with chaos if they do not approve the document; the voters do not know what document will be offered next if they turn down the first that is offered; and the drafters (not in Iraq perhaps, but certainly in other settings) have no incentive to shoot for even more of a consensus than is required by the ratification rule.
In some situations, it might be better if we put less weight on the supermajority (two-thirds of voters, or nine of thirteen states using majority or supermajority rules themselves, or two-thirds of registered voters) and more on choices. Perhaps the drafters should offer not one document for up or down vote, but two or three constitutions with the voters choosing which they preferred. (This is complicated. We might need to use approval voting with some requirement that each submission be approved by x% in order to discourage strategic drafters from submitting one or two completely unacceptable documents.) Perhaps a subset of drafters (delegates or members of the initial parliament) of specified size could submit a proposed constitution, so tha a parliament or constitutional convention could in this way offer the voters two or three documents from which to choose. That would constitute a kind of final offer arbitration (the winner is the one voters like best, so there is a real incentive for each coalition of drafters to avoid extremism). In any event, when there is no stable status quo for a safe landing, a supermajority requirement of any kind may be a poor ratification rule. And of course a simple majority rule can lead to future instability and gives the drafters enormous power.