As far as I'm concerned, no one has PROVED anything yet.
jazer isn't arguing that you can change the shape of a muscle, he's arguing that you can emphasize a seperate head of the pectoralis major which, considering the only thing that cannot be argued against(anatomy and physiology), seems entirely possible. Will it make a big difference? Probably not, but that isn't what's being argued. In the same way, Belial has used arguments based on anatomy and physiology to support the other side(alot of the neural stuff I don't fully understand yet), but everyone coming in and saying naw there's no upper and lower chest, you can't emphasize either, just because that's been the mantra here for years doesn't really contribute to the discussion.