In 2004, using the memorable experiment above, psychologists Daniel Simons and Christopher Chabris showed that when viewers are concentrating on a task they can become functionally blind to unexpected objects and events.

Remarkably, Cambridge-based parapsychologist A.D. Cornell had earlier demonstrated a similar effect in an even stranger experiment. In 1959 he dressed up in a white sheet and “haunted” a cow pasture near King’s College; none of approximately 80 passers-by seemed to notice anything at all. When he moved the “ghost walk” to the graveyard of St Peter’s Church, 4 of 142 people gave any clear indication of having seen the “experimental apparition,” and none of them thought they’d seen anything paranormal. (One saw “a man dressed as a woman, who surely must be mad,” one saw “an art student walking about in a blanket,” and two said “we could see his legs and feet and knew it was a man dressed in some white garment.”)

The following year Cornell again dressed as a ghost and passed twice across the screen in an X-rated cinema. 46% of the respondents failed to notice his first passage, and 32% remained completely unaware of him. One person thought she’d seen a polar bear, and another reported a fault in the projector. Only one correctly described a man dressed in a sheet pretending to be a ghost. Cornell concluded that the number of “true” hauntings may be grossly under-reported.

(A.D. Cornell, “An Experiment in Apparitional Observation and Findings,” Journal of the Society for Psychical Research 40:701, 120-124; A.D. Cornell, “Further Experiments in Apparitional Observation,” Journal of the Society for Psychical Research 40:706, 409-418.) (Thanks, Elizabeth.)