Update: Since we published this story yesterday, the Metropolitan Police has reported that a CS canister has been found at Heathrow T4. The supposition by the Metropolitan Police that it was a case of mass hysteria appears to have been inaccurate.

Heathrow’s Terminal 4 was evacuated on September 8 as fire crews were called in to investigate “possible hazardous materials” at the London airport. After a few hours of halted flights and frustrating inconvenience, emergency services declared that no “adverse substance” had been found anywhere in the airport.

People were allowed back into the terminal, and normal service was resumed. In the meantime, however, 21 people were treated at the scene by the London Ambulance Service. So what really happened at Heathrow?

According to the Metropolitan Police, it was probably “mass hysteria”. Such outbreaks – variously called mass psychogenic disorder, mass sociogenic illness, epidemic hysteria or mass hysteria – are all types of social contagion. They are typically characterised by the rapid spread, between members of a social group, of symptoms that have no apparent known cause and for which no physical infectious agent can be identified. The symptoms are real, but the trigger is psychological.

History is full of examples. In 1962, a textile factory in the US city of Spartanburg, South Carolina, shut down after dozens of workers reported........

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