A tin-foil hat, also tinfoil hat, is a general term for a piece of headgear made from one or more sheets of tin foil, aluminium foil, or other similar material. Some people wear the hats in the belief that they act to shield the brain from such influences as electromagnetic fields, or against mind control and/or mind reading. Hats made from foil are very rarely used, since the injuries they might guard against are highly speculative, and their effectiveness in preventing such harm would be dubious even if the danger were plausible. Instead, the concept has become a popular stereotype and term of derision; in Internet culture, the phrase (sometimes as the abbreviation "TFH") serves as a byword for paranoia.

Tin-foil hats and mental illness

There have been some people who believe in the efficacy of tin-foil hats and similar devices. Reasons for use include preventing abduction by alien beings, or stopping unpleasant experiences such as hearing voices in one's head. This draws on the stereotypical image of mind control operating by means of ESP, microwave radiation or other technological means. In some cases, belief in tin-foil hats could be a manifestation of a disorder such as paranoid schizophrenia.

The delusion of "mind control rays" or other invasive mental activity may seem very real to those afflicted with severe paranoid delusions, and such persons have been known to make and wear improvised

Scientific basis

There is a small amount of truth or reason to be found in the tin-foil hat story. A well constructed tin-foil enclosure would approximate a Faraday cage, reducing the amount of (notionally harmless) radiofrequency electromagnetic radiation inside. A common high school physics demonstration involves placing an AM radio on tinfoil, and then covering the radio with a metal bucket. This leads to a noticeable reduction in signal strength. The efficiency of such an enclosure in blocking such radiation depends on the thickness of the tin-foil, as dictated by the skin depth, the distance the radiation can propagate in a particular non-ideal conductor. For half-millimeter-thick tin-foil, radiation above about 20 kHz (i.e., including both AM and FM bands) would be partially blocked. (Classical Electrodynamics, John David Jackson, Wiley Press 1998.)

The effectiveness of the tin-foil hat as an electromagnetic shield for stopping radio waves is greatly reduced by the fact that it is not a complete enclosure. Placing an AM radio under a metal bucket without a conductive layer underneath demonstrates the relative ineffectiveness of such a setup. Indeed, because the effect of an ungrounded Faraday cage is to partially reflect the incident radiation, a radio wave that is incident on the inner surface of the hat (i.e., coming from underneath the hat-wearer) would be

A (somewhat tongue-in-cheek) study by graduate students at MIT determined that a tin-foil hat either amplified, or attenuated, incoming radiation between 1.2 and 2.6 GHz by as much as 20 dB, depending on frequency ; the effect was observed to be roughly independent of the relative placement of the wearer and radiation source. Note that GHz wavelengths are well below the putative skin-depth of even the thinnest foil.

Tin-foil hats in pop culture

• The paranoid centaur Foaly, in Eoin Colfer's Artemis Fowl series of books, wears a tin-foil hat to protect from mind-readers.
• In the Simpsons episode "Brother's Little Helper" Bart wears a tin-foil hat to protect himself from the influence of Major League Baseball's mind control satellite after he begins taking an anti-ADD drug "Focusyn".
• In the film Lovesick, Dudley Moore plays a psychiatrist who gives a homeless patient some aluminium foil to "protect" the patient from the "mind control rays" his patient claims are bombarding him.
• In Signs, the children and younger brother of the lead character wear tin-foil hats to prevent their minds from being read. This is parodied in Scary Movie 3, in which the tin-foil hats are actually giant Hershey's Kisses
• In the X-Men movies, Magneto wears a helmet that prevents Stryker's son (in X2) and Professor X (in X-Men and X2) from using telepathy against him.
• In the Sci Fi Channel (United States) original movie Control Factor, nonconsensual test subjects in a government experiment to incite violence in an inner city neighborhood use hats lined with copper wool to protect themselves from microwave-based mind control signals.
• Former Marvel Comics supervillain Juggernaut wore a helmet made of a mystical alloy as protection against telepathy.
• Tin-foil hats are often referenced on Internet forums such as Slashdot, Fark.com and Footballguys.com.
• In his autobiography, Frank Zappa refers to Echo Park residents "Crazy Jerry," an electricity addict and speed user who had been institutionalized a number of times, and his roommate known only as "Wild Bill the Mannequin Fucker," a chemist and methamphetamine cook who modified department store mannequines for sexual uses. Both are reputed to have employed metallic hats (vegetable steamers, aluminium pots, tin-foil etc.,) to "keep people from reading their minds." Zappa once recorded Crazy Jerry's life story, excerpts of which can be heard on some of his early recordings.
• The character Joe Wickes was featured in British soap EastEnders suffering mental illness and constructing a tin-foil hat for "protection".
• The novel Idiots in the Machine by Edward Savio portrays a character who believes that tin foil keeps harmful gamma rays away and becomes a media sensation, marketing a successful line of foil hats to Chicago.
• In Friends, Lizzie is a homeless woman who offers Phoebe her tin-foil hat.

See also

• List of hats and headgear
• Microwave auditory effect
• Tinfoil Hat Linux
• Homokaasu
• Slashdot subculture
• James Tilly Matthews
• Francis E. Dec