I always though it was based on physical anthropological reasons since when a "white" and "black" person produce a child together, its usually the black traits that will dominate more since they tend to be dominant. So "racially" the person appears black more often then not. Same thing sometimes happens when East Asian person mixes with white. But really what do you mean by one drop exactly? If we're using black as a way to define someone racially/physically then its usually what the person's physical appearance suggests. Case in point I know someone who is "white" and has a "drop" of native american blood in him but I would in no way consider him native based on his physical appearance.