In an exchange in the comments at a Chris Dillow post, I said:
Legitimacy for governments comes from citizens. Every citizen must be entitled to vote. That's the end of it.Tim Worstall replied:
Tsk. Every adult citizen, every sane citizen, perhaps, but even you don't believe "every".But funnily enough I do. I also think prisoners should be able to vote. I don't include children in this definition of citizen, so yes, every adult - but every adult.
The principle that governments derive their legitimacy from citizens is vital, I think. Where else could legitimacy come from? And that means the legitimacy of government authority over me personally derives exclusively from my vote. This applies to prisoners and the insane, and even to peers of the realm, just as much as everybody else, but with perhaps greater urgency in the first two of those instances because the state is exercising, or is likely to exercise, some form of compulsion over the people concerned. That compulsion needs to be legitimate.
There are also unfortunate precedents. It wasn't unknown for inconvenient dissidents in places like the Soviet Union to be declared insane. Excluding the insane from voting is potentially dangerous.
And if the removal of the vote from the insane is done on the grounds of mental competence, why should that principle stop there? Why not extend it to the educationally otherwise-able? How many voters are genuinely competent to form judgements on the whole range of policies we are confronted with?
On the other hand, what are the dangers of a genuinely universal franchise? That people with propeller hats might be elected to Parliament? That a "legalise burglary" party might be elected by convicts? I doubt either of these things would happen, and don't think there is really a downside at all.