If stuttering isn't psychological, than why don't most of us stutter while drunk, or on mdma?

If stuttering isn't psychological, than why don't most of us stutter while drunk, or on mdma? When i'm drunk i don't give a fuck and i have confidence, that somehow make my stutter so much better and i hardly ever stutter while drunk. I have tried MDMA once and i didn't stutter the whole night, and i talked to A LOT of people. And this is coming from a guy whos been stuttering since he was 6. Does anyone know why this happens?