I suspect the reason that the theory has never been really tested is that those who travel in the same circles I do are unwilling to do a true scientific test, as it would mean depriving birds of something they need to see if they would self correct. The closest they've come is to test chicken preference when it comes to meals. When I can get my cracked grain feed, it only has two powdery substances in it: Fish Meal and the vitamin pack. After 10-15 minutes, all the powders are gone out of my trough. Others use grain meals and in the same time frame, have found significant quantities of powder/meal in their trough. When tested, the meal was absent of fish meal and vitamin pack ingredients and almost entirely grain meals. This is far from scientific, but certainly makes one ponder.