The 21 grams experiment that tried to weigh a human soul

Written by Leah Hudson on September 26, 2025. Posted in Current News

It’s a little complicated to weigh a dying person on a hospital bed, but that didn’t matter to Duncan MacDougall

In the early 20th century, MacDougall’s unique, purpose-built scale was ready to receive test subjects.

One of the first, a tuberculosis patient described by The New York Times as an ordinary man with a “usual American temperament” was placed on the bed as he neared death.

With MacDougall and doctors watching over, the man died, and MacDougall noticed the scale’s counterweight dropped with surprising quickness.

The scales displayed the weight that had been lost with death: three-fourths of an ounce, 21 grams. MacDougall had his result, the weight of the human soul. Or so he thought.

MacDougall hoped to discover whether a soul had mass with his macabre experiment, and if so, how much. His tests with patients, later known as the 21 grams experiment, were limited in sample size and poorly designed.

Nonetheless, the experiment has endured through the decades, spawning movies, books, television episodes, and recreations. Part of why MacDougall’s experiment persists is because it attempts to provide hard proof of the ephemeral.

In the same way that people look for scientific proof of heaven or of an afterlife, MacDougall asked a question that has occupied philosophers and scientists for millennia. The 21 grams experiment reminds us how many of life’s mysteries remain unresolved.

MacDougall was working in the time of Spiritualism, a belief that a person’s spirit persists after death and is able to be contacted—sometimes in bizarre ways. Scientists and doctors, as well as mediums and theologians, carried out numerous experiments and parlour tricks to establish a relationship between spirit and matter.

Published in 1907 in the Journal of the American Society for Psychical Research, MacDougall’s paper was named “Hypothesis Concerning Soul Substance Together with Experimental Evidence of the Existence of Such Substance”.

One of his key ideas was that if personality or consciousness persisted beyond death, it must occupy some physical space in the body. To him, it was impossible that a soul could be nothing but ether.

Therefore, he decided, the mass of the soul would disappear from the body upon the moment of death, and he was determined to prove it.

Oddly enough, MacDougall was not a spiritual person. The Haverhill Evening Gazette described him as “hard-headed and practical,” with a scientific mind and a disinclination to believe in Spiritualism or psychic phenomena.

“He doesn’t sound like the kind of woo-woo, tarot-card reading, mystic type,” says bestselling science writer Mary Roach, author of Spook: Science Tackles the Afterlife.  At the time of his experiment, MacDougall was also described by The New York Times as a “reputable physician,” who had been working in the field for years.

How did the 21 grams experiment work?

For the experiment, MacDougall created a set of extremely sensitive scales to weigh the dying. He knew weight loss could occur after death come from breath leaving the body, the evacuation of the bowels or bladder, as well as sweat evaporation, so he tested and accounted for these issues.

For example, he had calculated that sweat evaporates “at the rate of one-60th of an ounce per minute.” He also weighed himself breathing in a full breath of air, as well as expelling it, neither of which had any effect on the scales.

Each of MacDougall’s six patients, five men and one woman, still lost a sudden, small amount of weight at the moment of death, a result that confirmed to MacDougall the soul’s departure from the body.

He noted that for some patients it was difficult to measure weight loss accurately, as one patient died while he was adjusting the scales. In addition, there was a “good deal of interference by people opposed to our work,” wrote MacDougall.

In his paper, he described this interference only as “friction on the part of officials of the institution,” without elaborating further.

MacDougall knew he was putting his reputation on the line by conducting this experiment. “It was very courageous what he did,” says Roach. After his experiment was published, The New York Times reported on it in a story entitled “Soul Has Weight, Physician Thinks”.

After this publicity around the experiment, MacDougall faced harsh criticism. While he did have some support from religious-minded people, scientists found his sample size too small, methodology flawed, and results inconsistent.

The Journal of the American Society for Psychical Research included an addendum to the publication of the experiment, explaining that so many newspapers had “misunderstood and misrepresented” it, that MacDougall had been afforded the opportunity to explain himself.

Even MacDougall wrote in his explanation that “I am well aware that these few experiments do not prove the matter any more than a few swallows make a summer.”

The doctor’s work continues

After the experiment, MacDougall worked on other ideas, including using X-rays to photograph the human soul. A 1911 story in The New York Times, entitled “Picturing the Soul,” explained that MacDougall had copied the experiment of medical electrician Dr. W. J. Kilner, known for his studies on human auras.

MacDougall intended to photograph and display the soul and described its presence on the resultant X-ray slides as “a light resembling that of the interstellar ether.” According to the Times article, MacDougall believed that photographing the soul is not hard, that “anybody can do it.”

Nine years later, MacDougall died at age 54, leaving behind far more questions than answers.

“He was definitely one of a kind,” says Roach, “In a way, he was the best kind of scientist because he was curious, and he didn’t really care what other people were going to say or think.”

Even 100 years later, MacDougall’s experiment continues to surface.

The scientific legacy of MacDougall’s work

Not long after MacDougall’s 21 grams experiment, a scientist named Professor Twining attempted to disprove MacDougall by weighing dying mice. He stated that the loss of weight at death was “due to either moisture, gas, or some substance we know nothing about.”

His assertions were published in The Los Angeles Herald in an article entitled “Soul Weight Theory is Now Disproved.”

A few decades later in the 1930s, R. A. Watters used a sealed chamber to examine animals at the moment of death, out of which (apparently) came a cloud of vapor thought to be the “immaterial body” of the soul.

After a lull in interest for nearly 70 years, another recreation was carried out in 2001 by physicist and endurance athlete Lewis Hollander, who tried to measure whether sheep lose weight when they die.

In 2005, physician Gerard Nahum proposed another experiment. Nahum suggested surrounding the body with electromagnetic detectors to detect energy (or a soul) escaping, though the study was never conducted.

Lights, camera, 21 grams

Beyond scientific research, the 21 grams experiment has also found an enduring home in pop culture. One of the most well-known instances of the experiment is in the 2003 thriller 21 Grams.

Although the movie offers little more than a reference to the experiment’s moniker, it does grapple with questions of death and loss.

Dan Brown, author of The Da Vinci Code, also nods to the experiment in his book The Lost Symbol. The book features a character who weighs a dying man in a chamber, and finds that the weight on the scales drops with his death.

The 21 grams experiment’s influence has even reached as far as Japanese manga: In one episode of the One Piece series, a scientist suggests souls weigh 21 grams. But, like with MacDougall, the One Piece scientist’s theory is eventually debunked.

MacDougall’s 21 grams experiment continues to persist because it begs a question many of us wonder: Are we more than just the body? Despite advances in science, the mystery of consciousness has not yet been unravelled.

Bruce Hood, professor of developmental psychology at the University of Bristol in the UK and author of SuperSense: Why We Believe in the Unbelievable, says that most people believe that our soul, consciousness, or “selfness” is separate from our brain or body, despite evidence that our brain produces these phenomena.

These beliefs are fueled by the idea that some part of us endures after death, but “people have to apply faith to believe in that,” says Roach. “We like to say it’s the soul or the personality and it’s us frolicking in the clouds, but that’s just hope. It’s just what people want to believe.”

The 21 grams experiment makes us wonder, Hood says. “People need the profound and the transcendent” to give them meaning in life. Hollander, the sheep-weighing scientist, as written in Roach’s book Spook, expressed a similarly profound sentiment:

“I think that at the moment of death that little window opens up. I think that maybe we’re all connected to something bigger than we are.”

Being human means never quite shaking our curiosity about the profound and the mystical.

While MacDougall’s experiment was deeply flawed, the idea behind it is appealing, and it continues to endure in the form of movies, books, anime, and even in this article.

We keep coming back to the 21 grams experiment because we’re still looking for the answer to MacDougall’s question.

Does any part of us continue after death? Maybe, one day, someone will find out.

See more here popsci.com

Please Donate Below To Support Our Ongoing Work To Defend The Scientific Method

PRINCIPIA SCIENTIFIC INTERNATIONAL, legally registered in the UK as a company incorporated for charitable purposes. Head Office: 27 Old Gloucester Street, London WC1N 3AX.