Ancient and quaint seem the days of witch crazes, demon scares and tulip manias. Instances of mass hysteria may strike you as rare events in modern advanced societies. But such outbreaks are products of their times. They’re still around today, just in different guises.

Aided and abetted by its status as an internet meme, the myth of an evil, supernatural Slenderman has been panicking adolescents since 2009, even culminating in an attempted murder by proxy. If it’s easy to brush this off as a case of impressionable teens with too much internet access, then what of otherwise rational late 20th-century American adults participating in suicide cults, Puerto Rico’s mythical cattle-killing Chupacabra monster, the “irrational exuberance” of the dot-com bubble in the 1990s, or the seemingly insane rush to make bad real estate investments in the latter 2000s?

A diplomatic dustup between the U.S. and Cuba may be the latest well-publicized case of collective delusion. In 2017, the U.S. State Department claimed its diplomats in Havana were subjected to “sonic attacks” that produced a range of physical symptoms including hearing loss, headaches and dizziness. Consequently, the federal government pulled out most of its embassy staff and sent packing most Cuban diplomats stationed in the U.S.

Although medical exams have identified unusual physical conditions in some diplomats, those exams lacked proper experimental controls and fall well short of providing evidence for any sort of sonic attack. There remains no demonstrably valid evidence that diplomats were subjected to sonic attacks at the American embassy in Havana – and a good deal of evidence has now been amassed suggestive of the contrary. The latest culprit to be fingered is the chirping of crickets or cicadas – in conjunction with mass hysteria.

So how do otherwise logical and informed 21st-century people fall under the spell of these mass delusions? Over the past several decades, psychologists and sociologists have used examples like these to dig into when and how this kind of false belief gains traction.

A recipe for collective delusion

Collective delusions are the culprits behind mass hysterias and related phenomena. As traditionally defined, they’re characterized by a rapid, spontaneous and temporary spread of false beliefs within a circumscribed population.

Nowadays that circumscribed population can be a virtual one, bounded only by cyberconnections to a shared source of misinformation. The recent upsurge in vocal flat-Earth proponents, for example, is not the result of geographical neighbors whipping each other into a near frenzy. Social media makes it easy to find like-minded others, serve distorted information to the curious, and stir up excitement about events such as the 2017 eclipse, celebrity endorsements, and a proposed rocket launch by a flat-Earth proponent intended to prove once and for all that we are all living on a disc.

Collective delusions emerge under a combination of several conditions. Each of these precursors is straightforward enough, but it’s harder to foresee when they might occur in concert. In turn, this makes predicting delusional outbreaks a very inexact science.

The most obvious precursor is the presence of multiple people who are sufficiently connected so as to share information or experiences.

Second, just as an isolated individual may develop some beliefs and behaviors that depart from prevailing norms, collective delusions and responses are more likely to occur in relatively insular groups or networks.

Third, a collective delusion is more likely to take hold if the group is undergoing some kind of distress. This could be rising unemployment, political destabilization or an enemy’s threats of warfare. On a smaller scale, a town may lose a crucial employer, or a fire-and-brimstone minister can instigate a satanic panic with rumors of baby-killing cults.

And fourth, the stressors are potent enough to trigger, in at least some individuals, either a psychosomatic response or scapegoating behavior. Psychosomatic reactions – physical symptoms with psychological causes – may be as mild as itching or as severe as blindness. Scapegoating involves blaming a group of innocent (or possibly nonexistent) others for causing problems – psychosomatic or otherwise.

When conditions are ripe, this catalyzing subset of group members sets off a chain reaction. They begin to seek and identify external causes for their distress, or sources for its relief. Psychosomatic responses spread; contempt for the scapegoats grows. People become hypervigilant and toss critical thinking out the window, looking for and finding imagined threats. Conspiracy theories are spawned, angels and demons invoked, fears stoked, panic induced. The supernatural may start to seem natural.

As more and more group members become ensnared in a positive feedback loop, the perceived threat is legitimized, only broadening and deepening social distress further. Because they are inherently newsworthy, mass delusions are picked up by mass media, which fan the flames even more.

In these ways, a nonexistent threat can set off a self-sustaining cascade of irrationality that lasts until the perceived threat recedes.

Delusion everywhere, to different degrees?

While descriptions of mass hysterias make great reading, they represent only the far end of a continuum of what sociologists like me call social diffusion processes. For the most part, these are quite mundane – you might recognize a few from your own daily life. While around the world stock market bubbles and bank runs make news, less frenetic responses to perceived threats and conspiracies abound: the 9/11 “truthers,” the recent uptick in flat-Earth beliefs, fears of gluten and genetically modified foods, climate change deniers, wars on science on some liberal college campuses, and more. Even the desire to be fashionable can be seen as a response to the fear of being excluded.

Simple mathematical equations can quite elegantly describe the speed, duration and extensiveness of the spread of beliefs and behaviors. A typical “diffusion model” shows how the penetration through a population of such things as beliefs, behaviors, illnesses, innovations or products is determined by just a few parameters. These typically include the group’s size, the density of its members’ interconnections and the inherent contagiousness of the thing being spread.

Irrational beliefs, and the often ill-considered responses they engender, can spread like an infection across groups as large as nations or as small as nuclear families. Sunshine, as they say, is the best disinfectant. Social impact theory would suggest that the best approach to administering social disinfectant is via large numbers of geographically nearby, authoritative nonbelievers.

In the case of the supposed sonic attacks in Cuba, one approach to stemming the scare would have been a rapidly deployed on-site investigation by acoustic experts, neurologists, psychiatrists and military strategists. A folklorist as well wouldn’t hurt. Short of such a full-frontal counterattack, disseminating easy-to-digest skeptical information as early as possible in the process should help to slow the diffusion process and quell a mass delusion.

It’s easy enough to be caught up in a mass delusion. Fads and fashions are great examples, though their most harmful consequence may be our embarrassment when we look back on some of our previous style choices. As long as people are stressed and living in groups, most of our mass delusions will remain invisible to us until they have already run their course.

This is an updated version of an article originally published on Dec. 18, 2017.