Mass hysteria at Heathrow airport – how social contagion works

By Kit Yates for The Conversation

Bath (UK): 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 rashes, numbness, nausea and fainting. Investigators suspected an insect in a shipment of cloth, but no evidence of such a cause was ever found.

Sociologists later concluded that, while an insect bite may have triggered the first case, the rest were probably psychogenic (something that originates from psychological factors rather than a physical cause).

Clusters of illness followed social ties, and the main predictors were background anxiety and stress – classic conditions for hysterical contagion.

Mass psychogenic effects have been recorded even further back in time. The infamous “dancing plague” of 1518 in Strasbourg began with a single woman dancing without pause. Within weeks, hundreds of others had joined her.

In a misguided attempt to help the victims “dance away their mania”, officials in the town hired musicians. They erected an enormous stage for the merrymakers to help them burn off their energy. Unsurprisingly, this only attracted more people to the fray. At its height, 15 people a day were reported to be dropping dead until the dancing abruptly stopped.

Positive feedback loop

In their early stages, infectious diseases typically spread according to a mathematical mechanism known as a positive feedback loop. These are characterised by a signal that triggers a response – or series of responses – which ultimately ends up amplifying the original signal.

In an epidemic, infected individuals can come into contact and infect susceptible people, creating more infectious individuals who have the power to infect more people, and so on.

Something similar happens in the spread of social epidemics – only in these cases, the illness is spread by the infectious power of emotion, rather than something physical. The same mathematics that we use to describe the explosive onset of an infectious disease can be used to describe the viral outbreak of an idea.

Just because an illness is spread by an idea or emotion, rather than a virus or bacterium, it doesn’t make that illness any less real for the communities or people affected.

Scientists have suggested that a hugely diverse range of social phenomena – from generosity to violence and from kindness to unemployment – may be socially contagious.

Some scientists have even come full circle by suggesting that diseases like obesity, which is typically considered to be a non-communicable disorder, may have a strong social component that allows it to spread like a contagious disease.

Whether teen pregnancy, for example, is genuinely socially contagious, as some scientists claim, is still hotly debated.

What is clear is that positive feedback loops can amplify an initially small quantity to unexpected magnitudes. For this reason, the impact of positive feedback is sometimes referred to as the snowball effect. A small amount of snow that begins rolling down a hillside picks up more snow as it rolls and increases in size.

The bigger it gets, the more snow it picks up, until the initially small snowball has gathered both size and pace.

It seems that social contagion, mediated by a positive feedback loop, may have been the cause of the disruption at Heathrow airport.

Of the 21 people assessed by ambulance staff, all but one were discharged at the scene. The Metropolitan Police even used the positive feedback loop terminology, suggesting the incident may have started with a single person falling ill and then “snowballed” from there.

The situation at Heathrow was quickly resolved, but when ideas spread like diseases, they’re much harder to stop than actual germs. Underestimating an idea’s potency, its longevity and its ability to enthral can lead us to misjudge or misunderstand how a situation will unfold.

One only has to look at the pervasive spread of disinformation throughout the COVID pandemic to see the damage that dangerously incorrect ideas – overstating the potential harms of effective vaccines, underplaying the risks of contracting COVID and falsely claiming the effectiveness of unproven treatments – can do.

The viral spread of such falsehoods through social media means they can reach far and wide in virtually no time – and are, consequently, extremely difficult to counter. We underestimate the snowballing of these pervasive myths at our peril.

Kit Yates is a Professor of Mathematical Biology and Public Engagement, in University of Bath.