A tin foil hat is a piece of headgear made from one or more sheets of aluminum foil or similar material. Alternatively it may be a conventional hat lined with foil. One may wear the hat in the belief that it acts to shield the brain from such influences as electromagnetic fields, or against mind control and/or mind reading; or attempt to limit the transmission of voices directly into the brain.
The concept of wearing a tin foil hat for protection from such threats has become a popular stereotype and term of derision; the phrase serves as a byword for paranoia and persecutory delusions, and is associated with conspiracy theorists.
The concept was mentioned in a science fiction story by Julian Huxley, "The Tissue-Culture King," first published in 1927, in which the protagonist discovers that "caps of metal foil" can be used to block the effects of telepathy.
Since then, the usage of the term has been associated with paranoia and conspiracy theories. The supposed reasons for their use include the prevention of perceived harassment from governments, spies or paranormal beings. These draw on the stereotypical images of mind control operating by ESP or technological means, like microwave radiation. The effectiveness of tin foil hats is disputable; however, the belief in their necessity is popularly associated with paranoia.