Duncan MacDougall, a physician from Haverhill, Massachusetts, hypothesized that souls have physical weight, and attempted to measure the mass lost by a human when the soul departed the body. MacDougall attempted to measure the mass change of six patients at the moment of death. One of the six subjects lost three-fourths of an ounce (21.3 grams).

MacDougall stated his experiment would have to be repeated many times before any conclusion could be obtained. The experiment is widely regarded as flawed and unscientific due to the small sample size, the methods used, as well as the fact only one of the six subjects met the hypothesis. The case has been cited as an example of selective reporting. Despite its rejection within the scientific community, MacDougall's experiment popularized the concept that the soul has weight, and specifically that it weighs 21 grams.

Although MacDougall's hypothesis was rejected, we can still assume the weight of the human soul to be 21grams