Dr. Gallagher is a board-certified psychiatrist and professor at Columbia University. He has been a practicing psychiatrist for over 25 years. He is the longest-standing American on the International Association of Exorcists, where he served as scientific advisor (and the only layman) for its governing council. He has personally witnessed over 150 cases involving phenomena for which there is no plausible alternative diagnosis than demonic possession.
Read his book Demonic Foes, My Twenty-Five Years as a Psychiatrist Investigating Possessions, Diabolic Attacks, and the Paranormal, published by Harper Collins. Available on Amazon, here.