The term "out-of-body experience" was introduced in 1943 by G. N. M. Tyrrell in his book "Apparitions", and was adopted by researchers such as Celia Green, and Robert Monroe, as an alternative to belief-centric labels such as "astral projection" or "spirit walking". OBEs can be induced by traumatic brain injuries, sensory deprivation, near-death experiences, dissociative and psychedelic drugs, dehydration, sleep disorders, dreaming, and electrical stimulation of the brain, among other causes. It can also be deliberately induced by some. One in ten people has an OBE once, or more commonly, several times in their life.