This raises an interesting question: What is a population measure of sanity?
The quality of the society and the government.
"Every nation gets the government it deserves." -- Joseph de Maistre
What do you mean by "the quality of the society"? I worry that this will end up being a circular definition or something very like one.
(Some plausible definitions clearly don't work. Imagine two societies with the exact same people, but in countries with very different natural resources. One may be much richer than the other as a result, leading to e.g. better healthcare, better education, less poverty, etc. So if those are part of what "the quality of the society" means then you're basically declaring richer populations as more sane even if they're composed of the same people.)
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