"but the idea is that a pigovian tax is successful even if the use doesn't go down significantly" That's why I said its political. If you're pro $TAX you say i

"but the idea is that a pigovian tax is successful even if the use doesn't go down significantly" That's why I said its political. If you're pro $TAX you say its pigovian and even if it doesn't 'work' you can claim its not supposed to. Whereas if you're anti $TAX, or want to appear tough on X, you say its a sin tax. I suppose you could argue a pigovian tax would have an upper bound, whereas a sin tax isnt bounded by anything, but for the types of things either label is used for, the externalities are so far removed and open to interpretation that you could still argue whether $TAX more than offsets/comes nowhere near offsetting X. Long story short, I don't think you could say definitively whether a tax is sin or pigovian.