Tin foil hat
A tin foil hat is a hat made from one or more sheets of aluminium foil, or a piece of conventional headgear lined with foil, worn in the belief or hope that it shields the brain from threats such as electromagnetic fields, mind control, and mind reading. The notion of wearing homemade headgear for such protection has become a popular stereotype and byword for paranoia, persecutory delusions, and belief in pseudoscience and conspiracy theories.
An early allusion to an "insulative electrical contrivance encircling the head during thought" appears in the unusual 1909 non-fiction publication Atomic Consciousness  by self-proclaimed "seer" John Palfrey (aka "James Bathurst") who believed such headgear was not effective for his "retention of thoughts and ideas" against a supposed "telepathic impactive impingement".
The usage of a metal foil hat for protection against interference of the mind was mentioned in a science fiction short story by Julian Huxley, "The Tissue-Culture King", first published in 1926, in which the protagonist discovers that "caps of metal foil" can block the effects of telepathy.
Some people have a belief that such hats prevent mind control by governments, spies, or paranormal beings that employ ESP or the microwave auditory effect. People in many countries who believe they are "targeted individuals", subject to government spying or harassment, have developed websites, conference calls, and support meetings to discuss their concerns, including the idea of protective headgear. Over time the term "tin foil hat" has become associated with paranoia and conspiracy theories.
Effects of strong electromagnetic radiation on health have been documented for quite some time. The efficiency of a metal enclosure in blocking electromagnetic radiation depends on the thickness of the foil, as dictated by the "skin depth" of the conductor for a particular wave frequency range of the radiation. For half-millimetre-thick aluminum foil, radiation above about 20 kHz (i.e., including both AM and FM bands) would be partially blocked, although aluminum foil is not sold in this thickness, so numerous layers of foil would be required to achieve this effect.
A belief also exists that aluminum foil is a protective measure against the effects of electromagnetic radiation (EMR) for many unspecified EMR frequencies. There are some allegations that EMR exposure has negative health consequences.
In 1962, Allan H. Frey discovered that the microwave auditory effect (i.e., the reception of the induced sounds by radio-frequency electromagnetic signals heard as clicks and buzzes) can be blocked by a patch of wire mesh (rather than foil) placed above the temporal lobe.
In art and media
Foil hats have appeared in films such as Signs and Futurama: Into the Wild Green Yonder. In the 2002 film Signs, the children and younger brother of the lead character wear tin-foil hats to prevent their minds from being read. 
The paranoid centaur Foaly, in Eoin Colfer's Artemis Fowl series of books, wears a tin-foil hat to protect from mind-readers.
The novel Idiots in the Machine by Edward Savio portrays a character who believes tin foil keeps harmful gamma rays away, becoming a media sensation after marketing a successful line of foil hats to Chicago.
- Electromagnetic hypersensitivity
- Electromagnetic radiation and health
- Electronic harassment
- Electrophonic hearing
- Electrostatic discharge materials
- Embassy attack accusations in Cuba (and China)
- Faraday cage
- The Hum
- Microwave auditory effect
- On the Origin of the "Influencing Machine" in Schizophrenia
- Sonic weapon
- "Foil - metallurgy". Encyclopædia Britannica. Retrieved 17 July 2016.
- Bathurst, James (1909). Atomic Consciousness Abridgement. W. Manning, London.
- Wilson, Daniel (June 2016). "Atomic-Consciousness". Fortean Times.
- p. 118
- Huxley, Julian (1925–1926). "The Tissue-Culture King: A Parable of Modern Science". The Yale Review. XV: 479–504.
- Huxley, Julian (August 1927). "The Tissue-Culture King". Amazing Stories.
Well, we had discovered that metal was relatively impervious to the telepathic effect, and had prepared for ourselves a sort of tin pulpit, behind which we could stand while conducting experiments. This, combined with caps of metal foil, enormously reduced the effects on ourselves.
- Weinberger, Sharon (14 January 2007). "Mind Games". Washington Post. Retrieved 29 June 2015.
- "Hey Crazy--Get a New Hat". Bostonist. 15 November 2005. Archived from the original on 3 May 2007. Retrieved 5 April 2007.
- Adey, W. R. (December 1979). "Neurophysiologic effects of Radiofrequency and Microwave Radiation". Bulletin of the New York Academy of Medicine. 55 (11): 1079–1093. PMID 295243.
- Jackson, John David (1998). Classical Electrodynamics. Wiley Press. ISBN 978-0-471-30932-1.
- Lean, Geoffrey (7 May 2006). "Electronic smog". The Independent. London. Archived from the original on 17 May 2008. Retrieved 9 June 2009.
- Frey, Allan H. (1 July 1962). "Human auditory system response to modulated electromagnetic energy". Journal of Applied Physiology. 17 (4): 689–692. doi:10.1152/jappl.19188.8.131.529. ISSN 8750-7587. PMID 13895081.
- Elder, Joe A.; Chou, C.K. (2003). "Auditory response to pulsed radiofrequency energy". Bioelectromagnetics. 24 (S6): S162–73. doi:10.1002/bem.10163. ISSN 0197-8462. PMID 14628312. Archived from the original on 4 November 2012.
- Media related to Tin foil hats at Wikimedia Commons