What exactly is Bestial supposed to represent, on a character with human IQ? It doesn't seem to make sense to me. Surely characters with enough intelligence to comprehend something that complex at all would quickly learn what was going on and how to deal with it, whatever they started out with.
I suppose an exception might be creatures that evolved as solitary animals, and were as intelligent as humans in most ways but didn't have as much in the way of brain structures specialised for analysing social interactions. Even there, though, in theory I'd have thought they could learn it mechanically, in the same way that a sufficiently intelligent autistic person can learn to mimic some of the social routines of neurotypical people "by numbers" even if they don't exactly understand why the people care, only that everyone's happier if you make an effort to get it right.
By the way, here's Bestial's old Disadvantage of the Week thread for quick reference.
http://forums.sjgames.com/showthread.php?t=153404 That thread pretty quickly came to the conclusion that Bestial won't necessarily mean the same thing for all characters - one species' ways might differ from humans in different respects from another's, leaving them with different sets of things they don't understand about humans.