" Scientific research" ? Citation needed.
It seems to me, having picked a few puppies in my life, at the time I was merely attempting to select for certain traits that might actually manifest in those 8 week old puppies, perhaps in one pup more than the others. i.e., prey drive as exhibited by chasing a ball or any moving object. If one has ever attempted to cut the nails of a puppy and had it put up an unusually ferocious fight, I think one is observing a trait that could be called temperament. Therefore, a reaction to the "muzzle shake" could also be a subjectively observable and measurable test compared to the other pups in that litter. Whether or not it would translate into the adult dog behavior is, it seems to me, a rather difficult thing to prove or disprove, "scientifically" or otherwise, because it can be modified or suppressed by so many experiences, i.e, training, etc. The fact is that people have been selecting puppies in various ways for certain traits, based on their own experience, for thousands of years. I think the proof is in the pudding. If it didn't work, they wouldn't still be doing it.