The genetics of mental illness. In particular, schizophrenia.
One of the reasons I’ve never had an urge whatsoever to ever try pot is because both my paternal grandmother and aunt were schizophrenic, with the latter in particular’s illness exacerbated by substance abuse. Which, yanno. It seems no one believes me when I tell them about the pot thing, until I follow it up with, “… because when my dad was a teenager his mentally ill, drug addict sister would set fire to the curtains in their house while everyone was asleep.”
I don’t want to be the next Aunty Tanya. If the illness manifests, I’d rather be Granny, who wasn’t violent, just… difficult. Ellipses intended.
Incidentally, we didn’t know about Granny until she died, and schizophrenia was mentioned on the death certificate. Apparently she’d been on medication for it most of her life, and had never told a single soul (or, at least, not her son or his family).
That’s stigma for you, I guess.