JFTDWS
+++ Out of Cheese Error +++
I suspect there's not much research out there on the subject because there's not enough money to be made from it. You could turn the question around and ask where the scientific evidence is that shows bits do not cause pain.
There's plenty of money in it - it's big business, equestrian sport. There is no evidence that bits cause pain - that's not the same as there being evidence bits don't cause pain, but that's virtually impossible to prove. Plenty of horses accept bits happily - mine do, and give no indication of pain when used appropriately. When someone abuses the contact, my 8 year old has no hesitation in letting them know - I've seen him do it. So I'm pretty confident he's happy with it the rest of the time. That doesn't mean that every horse will react to a snaffle or curb the same way - mouth conformation and history dramatically affect how accepting a horse is of the bit...