Studies that relate to the concept of souls include Dr. Duncan Macdougall's experiment published in 1907, Dr. Raymond Moody's interviews published in 1975, and Dr. Sam Parnia's experiment published in 2014. As of 2015, Parnia's experiment is considered the largest study on out-of-body experiences.
Macdougall's experiment involved weighing human and animal bodies before and after death. He based this on the assumption that souls are made of matter, and that God endows humans but not animals with souls. He observed a weight loss of 21 grams in humans, and no weight loss in dogs. Researchers don't consider his results credible because they were inconsistent, his measuring methods were imprecise, his sample size was small, and the experiment was never replicated.
Moody drew on anecdotal evidence for his study. He conducted more than 100 interviews with people who claimed to have had near-death experiences and found that the stories shared a few similarities. He is considered to have coined the term "near-death experience" and popularized the concept with his subsequent successful book, "Life After Life."
For his experiment, Parnia had hospitals install shelves with pictures on them that could only be viewed from the ceiling, thinking that someone who had an out-of-body experience would be able to verify an image. However, he had trouble collecting enough data, as only two of the 2,060 heart-attack patients he documented were able to report out-of-body experiences, and one of the two became too sick to continue interviewing. This patient did not have his heart attack close enough to one of the shelves with an image, although Parnia notes that he was able to describe accurately a few details about how the doctors treated him. Scientists have still not been able to demonstrate conclusively that near-death experiences and out-of-body experiences have a spiritual origin.