In 1907 Dr Duncan MacDougall weighed six people in a nursing home who were within a few hours of death from tuberculosis. After they had expired, he weighed the corpses. He claimed to find a small loss of mass in each case, which he ascribed to the soul's departure from the body. In his words:
The subject was that of a man of larger physical build, with a pronounced sluggish temperament. When life ceased, as the body lay in bed upon the scales, for a full minute there appeared to be no change in weight. The physicians waiting in the room looked into each other's faces silently, shaking their heads in the conviction that out test had failed.
Then suddenly the same thing happened that had occurred in the other cases. There was a sudden diminution in weight, which was soon found to be the same as that of the preceding experiments.
I believe that in this case, that of a phlegmatic man slow of thought and action, that the soul remained suspended in the body after death, during the minute that elapsed before it came to the consciousness of its freedom. There is no other way of accounting for it, and it is what might be expected to happen in a man of the subject's temperament.
The average weight of the soul, according to MacDougall, was 21 grams (hence the movie title).