July 1518 lives long in the memory of the people of Strasbourg. At the height of a hot summer a distraught French peasant stepped out onto the street, and spontaneously and fervently began to dance. In what must rank as one of the strangest social phenomena in history, she was soon joined by over 400 fellow dancers who, over the course of a month, were afflicted by similarly distressed, trance-like convulsions, dancing day and night, often without pausing to eat, drink or rest. Contemporary sources were unambiguous on the topic: cathedral sermons, physician notes and even documents created by the Strasbourg City Council each made specific reference to this ‘dancing’.  While local authorities looked on, bemused, many of the participants died, struck down by dehydration, heart attacks and sheer exhaustion.

Explanations for the dancing outbreak abounded. Concerned burghers soon discredited astrological or supernatural causes, finally settling on ‘hot blood’, a quasi-scientific explanation which argued that abnormal internal temperatures were disrupting the brain’s ability to function. Elsewhere , the local council constructed a wooden stage and employed musicians to keep the dancers moving, convinced by a belief that the mania could be expunged by repetition, a perverse and fatal move.

In the centuries that followed, the Strasbourg dancing plague became something of a fixation for scientists. In 1952, Eugene Backman posited a theory that the dancers were made manic by ‘ergot’, a mould that grew on the rye that made up much of the populations’ diet and when consumed had hallucinogenic effects similar to those caused by LSD. John Waller, in his A Time to Dance, A Time to Die  discredits this, rejecting the idea that the consumption of this substance could provoke “coordinated movements that last[ed] for days”. He argues that the dancing plague of 1518 was an instance of mass hysteria, the spontaneous manifestation of the same or similar hysterical physical symptoms by more than one person.

There are hundreds of recorded cases of mass hysteria, including the West Bank Fainting Epidemic – a 1983 incident in which Palestinian teenage girls on the Gaza Strip fainted en masse, struck down by a false rumour of ‘chemical warfare’ – or a 2007 incident in Mexico City in which a group of students at a boarding-school temporarily lost the ability to walk. It involves a collective belief in the presence of some ailment which induces tangible physical reactions; such reactions are later proven to be psychosomatic, conjured by a mutual pressure on the part of the victims.

Though the psychological cause of the Strasbourg case was eventually explained, attributing contemporary outbreaks of collective panic to irrational quasi-scientific explanations is not as archaic a tendency as one might expect. Whereas in accounting for the dancing plague Waller benefited from the comfortable objectivity afforded by centuries of hindsight, contemporary psychologists and scientists must wrestle with the expectations of a desperate and often belligerent body of victims eager to find a physical cause. In late 2011, twelve cheerleaders simultaneously developed Tourette’s-like symptoms in a school in LeRoy, New York, and were afflicted by tics and convulsions for many months. This resulted in a state-funded investigation, the first line of inquiry of which was to seek out pernicious bodies in the local environment. When no such bodies were found and the investigation returned a verdict of ‘mass hysteria’, parents of the afflicted girls were insulted and demanded a fresh report.

As Dr Phil Stringer, Senior Educational Psychologist for Hampshire County Council attests, “once you start talking about illnesses that are psychological in origin, people can get very defensive about it – there still seems to be some stigma attached to it, in a way that there isn’t with physical illness.” Moreover, the parents’ insistence on the presence of a physical irritant only served to validate the girls’ psychological instability in their own minds. As Stringer notes, “we have to remember that the parent-child dynamic is very powerful and in some cases adults can unwittingly talk their children into being ill.”

Stringer is actually surprised that instances of mass hysteria aren’t more common in educational institutions, arguing that the conditions are “ripe for outbreaks”. Schools seem to cultivate the ideal milieu for hysterical behaviour, defined as they are by high densities of impressionable young people who are being continually exposed to a cocktail of different social and academic pressures. Indeed, some of the more notable instances of mass hysteria have evolved in primary schools. The Tanganyika Laughter Epidemic of 1962, for example, saw the laughter of three girls in a boarding school spread rapidly to 95 of the 159 pupils and then on to surrounding villages, affecting communities in Uganda and Tanzania over the course of 8 months. Children returning home spread the hysteria to their families, damaging local economic and social stability.

In many of the school cases, the pattern by which the girls succumbed to the laughter is predictable: the behaviour of the eldest and most socially prominent girls informed that of the younger students. Stringer suggests this is an “identification process”, as younger, more impressionable students seek to emulate their older counterparts.

But why do these incidents attract such concerted interest from scientists, psychologists and the general public? What is it about mass hysteria that provokes such widespread inquisition? The phenomenon is appealing to the sane observer as an interesting oddity, an alien occurrence that is more suited to vaudeville theatre than everyday life. We are fascinated by the cult of mystery that surrounds it, the pervasive sensation of the farfetched and ridiculous, despite recent scientific explanations of its origin. It is, ultimately, interesting because it appears so very unusual, so inhuman, and because it does not appear to threaten us, the rational onlooker.

Such a condescending assessment may well be flawed. Mass hysteria is fascinating precisely because it is the exaggeration of a natural and undeniable human propensity – to be vulnerable to mass emotional excitement. Whether screaming Beatlemania or the reaction to the death of Princess Diana, modern society places individuals under pressure creating incidents of mass hysteria, just without the academic title. The mass outpouring of emotion in the week after Diana’s death was watched sceptically by many observers. Over one million bouquets were left at her Kensington home, while news reports showing men, women and children sobbing uncontrollably became commonplace.  The Telegraph’s Ed West argued that tributes to the late Princess turned into an “orgy of sentimentality” which created the impression that “an invading army had spiked the water supply and everyone had gone bonkers overnight.” Though West’s tone is jovial his point is clear: this sort of outpouring of collective emotion was a self-perpetuating frenzy rather than a considered phase of mourning, akin to episodes of mass hysteria.

What is evident is that any pretensions toward an objective, impartial perspective on mass hysteria are misguided – the academic validation afforded to the Strasbourg Dancing Plague or the Tanganyika Laughter Epidemic does not render such events as removed, unrecognisable social anomalies. Instead, these incidents, bizarre as they may seem, reveal some sad but enduring human fallibilities – under intense pressure, there is always the threat that humans may resort to panic, desperation and mindless conformity.