The year was 1374. In dozens of medieval towns scattered along the valley of the River Rhine hundreds of people were seized by an agonising compulsion to dance. Scarcely pausing to rest or eat, they danced for hours or even days in succession. They were victims of one of the strangest afflictions in Western history. Within weeks the mania had engulfed large areas of north-eastern France and the Netherlands, and only after several months did the epidemic subside. In the following century there were only a few isolated outbreaks of compulsive dancing. Then it reappeared, explosively, in the city of Strasbourg in 1518. Chronicles indicate that it then consumed about 400 men, women and children, causing dozens of deaths (Waller, 2008).Not long before the Strasbourg dancing epidemic, an equally strange compulsion had gripped a nunnery in the Spanish Netherlands. In 1491 several nuns were ‘possessed’ by devilish familiars which impelled them to race around like dogs, jump out of trees in imitation of birds or miaow and claw their way up tree trunks in the manner of cats. Such possession epidemics were by no means confined to nunneries, but nuns were disproportionately affected (Newman, 1998). Over the next 200 years, in nunneries everywhere from Rome to Paris, hundreds were plunged into states of frantic delirium during which they foamed, screamed and convulsed, sexually propositioned exorcists and priests, and confessed to having carnal relations with devils or Christ.These events may sound wildly improbable, but there is clear documentary evidence that they did in fact happen. The dancing plagues were independently described by scores of physicians, chroniclers, monks and priests, and for the 1518 outbreak we can even read the panicky municipal orders written by the Strasbourg authorities at the time of the epidemic (Midelfort, 1999; Waller, 2008). Similarly, trial documents and the archives of the inquisition provide copious, in-depth accounts of nuns doing and saying the strangest of things (Sluhovsky, 2002).Writers then and now have offered various interpretations of these strange and sometimes deadly crises. It has been suggested that the dancing maniacs of 1374 and 1518 were members of a heretical dancing cult. Contemporary observers, however, made clear their view that the dancing was a sickness. Nor did the Church, at a time when heresies were quickly suppressed, believe the dancers to be anything but victims of a terrible affliction, natural or divine. In recent decades a vogue for simple biological explanations has inspired the view that epidemic madnesses of the past were caused by the ingestion of ergot, a mould containing psychotropic chemicals (Backman, 1952; Matossian, 1989).But scholarship in the fields of psychology, history and anthropology provides compelling evidence that the dancing plagues and the possession epidemics of Europe’s nunneries were in fact classic instances of a very different phenomenon: mass psychogenic illness.Altered statesAn important clue to the cause of these bizarre outbreaks lies in the fact that they appear to have involved dissociative trance, a condition involving (among other things) a dramatic loss of self-control. It is hard to imagine people dancing for several days, with bruised and bloodied feet, except in an altered state of consciousness. But we also have eyewitness evidence that they were not fully conscious. Onlookers spoke of the dancing maniacs of 1374 as wild, frenzied and seeing visions. One noted that while ‘they danced their minds were no longer clear’ and another spoke of how, having wearied themselves through dancing and jumping, they went ‘raging like beasts over the land’ (Backman, 1952). The hundreds of possessed nuns described in chronicles, legal records, theological texts or the archives of the Catholic Inquisition were equally subject to dissociative trance (Newman, 1998; Rosen, 1968). Some may have simulated the behaviour of the demoniac as a means of eliciting positive attention (Walker, 1981), but the detailed descriptions of astute and cautious inquisitors leave little doubt that most were genuinely entranced.How might we explain these epidemics of dissociation? Ergot could have induced hallucinations and convulsions in nuns who ate bread made from contaminated flour, but it is highly unlikely that ergotism would cause remorseless bouts of dancing (Berger, 1931). Nor is there any evidence that what the victims of mass possession ate or drank made any difference. Rather, as explained below, there are very strong indications that fearful and depressed communities were unusually prone to epidemic possession. And given that there is a well-established link between psychological stress and dissociation, this correlation is immediately suggestive of mass psychogenic illness.Fear and loathingThe years preceding the dancing epidemics were exceptional in their harshness. The 1374 outbreak maps on to the areas most severely affected, earlier in the same year, by one of the worst floods of the century. Chronicles tell of the waters of the Rhine rising 34 feet, of flood waters pouring over town walls, of homes and market places submerged, and of decomposing horses bobbing along watery streets (Backman, 1952). In the decade before the dancing plague of 1518, famine, sickness and terrible cold caused widespread despair in Strasbourg and its environs (Rapp, 1974). Bread prices reached their highest levels for a generation, thousands of starving farmers and vine growers arrived at the city gates, and old killers like leprosy and the plague were joined by a terrifying new affliction named syphilis.These were intensely traumatic times. Nuns were protected from many of the indignities of daily life, but nunneries could also become toxic psychological environments. Even in well-managed communities, some nuns were inevitably unhappy. Sisters were often consigned to lives of quiet contemplation in accordance with the wishes of their parents rather than any conspicuous piety on their own part. Once inside the cloisters it was very hard for them to get out. But those who keenly embraced the spiritual life were often the most desperate. Tormented by a feeling of falling short of the exacting standards of holiness imposed by their orders, plenty reflected with terrible fear on the fiery destiny awaiting those impure in mind or deed.A notable example is that of Jeanne des Anges, Mother Superior of the Loudun nunnery in southern France, who became infatuated with a local priest, Father Grandier, in the year 1627. ‘When I did not see him’, she later confessed, ‘I burned with desire for him.’ In consequence, Jeanne felt overwhelming worthlessness and guilt. After weeks of painful penance and introspection, she fell into a dissociative state during which she repeatedly accused Grandier of plotting with Satan to make her lust after him. Within days, several more nuns had followed suit, all deliriously pointing the finger at the hapless priest. After an investigation by the Inquisition, Grandier was burnt alive (de Certeau, 2000). As in the case of the Loudun nunnery, a deep, guilty longing for human intimacy could trigger collective breakdowns. This is in part why, during their possession attacks, dissociating nuns often behaved with alarming lewdness: lifting their habits, simulating copulation, and giving their demons names such as Dog’s Dick, Fornication, even Ash-Coloured Pussy. Guilt and desire could drive a nun to distraction (Sluhovsky, 2002).The fortitude of many a nun was most severely tested during the evangelical reform movement that swept their communities from the early 1400s. Striving to restore the harsh spiritual codes of earlier centuries, reformers instructed the nuns to consume only the blandest fare, to spurn all vanity, to adopt exacting regimes of abstinence and self-abasement, and to meditate routinely on the evils of Satan and the flames of Hell. Often the younger daughters of nobles or rich burghers, many nuns did not adjust well to tasteless meals, pillow-less beds and evenings bereft of music and conversation. Hence the arrival of reformist Mother Superiors precipitated a significant number of mass possessions. Take, for example, the Ursuline nuns of Auxonne in eastern France who experienced a possession crisis in 1658 after the appointment of the evangelical Barbe Buvée to their nunnery. For several years, distressed and dissociating nuns accused her of being a witch, of killing babies and of being a lesbian. Barbe Buvée was exonerated but judiciously assigned to an alternative nunnery. The possession crisis petered out (Sluhovsky, 2002).Mass possession also affected secular communities, and here too the role of stress is abundantly clear. The girls whose ‘grievous fits’ and ‘hideous clamors and screeching’ set off the Salem witch panic in New England in 1692 were the members of a community rent by factional strife (Demos, 1983). They were also terrified of attacks by the Native American tribes which had already slaughtered the parents and relatives of several of those at the heart of the witchcraft accusations (Norton, 2003).Fear and anguish were the common denominators of dancing plagues and possession crises. But this is only part of the story.