However, the psychological dysfunction theory has been disputed by other researchers, including Gauld and Cornell who said the psychological tests employed were invalid. Psychiatrist Ian Stevenson proposed that spirits of the dead may account for more poltergeist activity than realized. In his study of a number of cases attributed to agents and to spirits of the dead, Stevenson noted significance differences. The phenomena in living agent cases was without purpose and often violent, while cases involving spirits of the dead featured intelligent communication, purposeful movement of objects, and little violence.