I guess I can't really address the main question in the OP but I do have a theory - I really feel like the water balloon/water/egg/whatever theory just addresses the behavior, not the cause. Rearing is one of those things that is so dangerous that, personally, I just don't think addressing the behavior is enough.
I've "solved" two rearers by addressing the issue - which, in both cases, was that neither horse would move off their riders leg when they got to a high enough level of arousal.
My mare Lacey was a rearer when I got her. Nowadays, she hasn't reared in years. She's been "triggered" where she wanted to rear but I trained her to deal with those feelings in other ways, by moving her feet instead of getting "stuck" and rearing, and she used that training to avoid going up.
I don't doubt that she would still go up if I kept her from moving forward and really goaded her into it, but I don't because...well, I'm not a jerk.
Anyway, I never recommend making the horse think it's hurt itself via the egg/water/etc. I think there are much more effective, long term, ways to solve rearing...
Fabio - 13 year old Arabian/Lipizzan gelding
Hazel - 14 year old Angora goat
Atticus - 4 year old LaMancha/Alpine cross goat
Rest peacefully, Lacey.
Last edited by Wallaby; 07-03-2012 at 11:53 PM.