As an aside, I've started quite a few fortresses now, usually in serene/joyous type biomes.
I recently decided to try a triple biome (untamed wilds, joyous, and haunted); the predominant biome (particularly the wooded area) was haunted.
It seemed like every time I turned around, someone or other was getting possessed. the frequency was really quite high. Quite a few woodcutters, hunters, and farmers (plant gatherers) were getting possessed too; about half the time while they were out in the woods. The map ended up turning downhill: I christened one of my hunters "Werewolf Slayer" after her second kill. She eventually became possessed during a hunt, couldn't complete, and went insane. The werewolves took the opportunity to kill the dogs i had and slaugther some farmers. Everyone was starving and complaining of thirst, and half the hill was ore - but no magma. At the time I abandoned, I had about 9 legendary artifacts after about 3-4 years. IIRC, only 1 was a non-possessed artifact.
Previously, possessions were extremely rare; most of the time I got Fey mood ("inspired" i guess; the 'regular' one where they mutter abotu what they need rather than sketch, and will become legendary in a skill), with the occasional withdraw from society. When I made a second fortress in a terrifying biome (volcanic & Aquifer, for kicks) I had a similar result. Very frequent possession of dwarves; one or two withdraw from society (compared to, at the time, 6 possessions) and no "inspired" fey mood.
It's my opinion (not substantiated fact) that the type of biome influences the type of Strange Mood you get, as well as possibly the frequency as well.
Your experience will vary; i'm interested to hear if anyone else has noticed any sort of correlation.