Mass psychosis is not a formal medical diagnosis. You won’t find it in any psychiatric manual. The term is used colloquially to describe situations where large groups of people seem to adopt irrational beliefs or develop unexplained physical symptoms at the same time, but the clinical reality is more specific and more interesting than the phrase suggests. What psychiatrists and public health researchers actually study are two related but distinct phenomena: mass psychogenic illness, where groups develop real physical symptoms with no identifiable cause, and shared psychotic disorder, where a delusional belief passes from one person to another in a close relationship.
Why “Mass Psychosis” Isn’t a Clinical Term
The DSM-5-TR, which is the standard diagnostic manual for mental health professionals, does not contain an entry for mass psychosis. The closest recognized condition is shared psychosis, formerly called folie à deux (French for “madness of two”). In shared psychosis, a person develops delusional beliefs after prolonged close contact with someone who already has a psychotic disorder like schizophrenia. The person with the primary disorder is typically the socially dominant one in the relationship and gradually convinces the other person of their unusual beliefs. Notably, the secondary person usually stops holding the delusion once they’re separated from the primary one.
That pattern works in small groups, usually families. It doesn’t scale to thousands or millions of people. When people use the phrase “mass psychosis” to describe entire societies going off the rails, they’re borrowing clinical language metaphorically. The psychological mechanisms behind large-group phenomena are real and well-documented, but they look quite different from individual psychosis.
Mass Psychogenic Illness: The Real Phenomenon
The phenomenon researchers actually study is mass psychogenic illness (MPI), defined as the collective occurrence of physical symptoms among two or more people in the absence of any identifiable pathogen or environmental cause. The symptoms are real. People genuinely faint, vomit, develop tics, or experience pain. But no virus, toxin, or structural problem can be found to explain them.
MPI does not hold the status of a formal diagnostic category, and it remains loosely defined beyond its general features. Some researchers have criticized the concept itself, arguing that labeling group behaviors as “mass hysteria” reflects a Western bias toward treating collective experiences as irrational or abnormal. Still, documented outbreaks are numerous and consistent enough to reveal clear patterns in who gets affected and under what conditions.
Historical Examples
Outbreaks of mass psychogenic illness span centuries and continents. During the medieval period in Europe, the so-called Dancing Plagues saw large groups of people dance uncontrollably until they collapsed from exhaustion, and in some cases, reportedly until death. No one has definitively explained what triggered these episodes, though extreme social stress and religious fervor are commonly cited.
The Salem Witch Trials of 1692 involved symptoms that some modern researchers attribute to MPI. Afflicted individuals reported contorting their bodies, suffering seizures, screaming in pain, hiding under furniture, barking like dogs, and seeing disembodied spirits. These symptoms spread through a tight-knit, high-stress community under enormous religious and social pressure.
More recent cases show the phenomenon hasn’t disappeared. In 2013, 18 girls at a high school in Danvers, Massachusetts, developed hiccup-like symptoms that escalated into high-pitched yelping sounds. No medical cause was identified. In 2021, a wave of “TikTok tics” emerged, with young people developing Tourette-like movements and vocalizations after watching videos of creators with tic disorders. Clinicians worldwide reported a sharp increase in functional tic-like behaviors, particularly among adolescent girls.
Havana Syndrome offers another lens on the phenomenon. Between November 2016 and June 2018, U.S. embassy personnel in Cuba reported dizziness, headaches, memory loss, tinnitus, fatigue, confusion, and difficulty walking. While some investigators initially suspected a directed energy weapon, subsequent analyses raised the possibility that at least some cases reflected psychogenic spread, where awareness of the symptoms in colleagues made individuals more likely to notice and attribute their own bodily sensations to the same cause.
How Symptoms Spread Through Groups
The psychological engine behind these outbreaks is emotional contagion, the automatic adoption of another person’s emotional state. This process often happens below the level of conscious awareness. When you see someone grimace in pain, your brain activates some of the same neural pathways that would fire if you were in pain yourself. Brain regions with mirror properties respond both when you perform an action and when you watch someone else perform it, creating a built-in mechanism for sharing emotional experiences.
The process works in stages. First, people unconsciously mimic the facial expressions, postures, and vocalizations of those around them. Then, that physical mimicry feeds back into their nervous system. Adopting a fearful expression, for example, produces subtle physiological changes that nudge you toward actually feeling afraid. This feedback loop is transmodal, meaning that witnessing just one element of someone’s emotional expression, like their facial tension, can trigger a full emotional response in the observer including changes in body language, voice, and internal state.
There’s also a conscious layer. People actively compare their own feelings to those of the people around them. In a high-anxiety environment where several people report feeling sick, you’re more likely to interpret your own ambiguous sensations (a mild headache, slight nausea) as symptoms of whatever is supposedly going around. This isn’t faking. The symptoms are genuinely experienced. The cause is psychological rather than infectious.
Conditions That Make Outbreaks More Likely
Documented MPI outbreaks cluster heavily in workplaces and schools. Both are formally structured environments with well-defined roles and rules, and participation isn’t truly voluntary. Financial necessity keeps people at jobs; legal requirements keep kids in school. When stress builds in these settings, people often perceive it as chronic and feel they have extremely limited avenues of escape.
Several specific stressors appear repeatedly in case studies. Boredom is one: outbreaks in industrial settings have consistently involved repetitive tasks performed at fixed workstations with fixed production paces. That kind of monotony is linked to muscular tension, depression, and a tendency to express psychological distress as physical symptoms. Production pressure compounds the problem. In the majority of documented industrial outbreaks, workers were under significant pressure to increase output, often with unwanted overtime on top of already demanding schedules.
Poor communication and isolation also play a role. In noisy work environments, workers reported feelings of loneliness because the noise made normal conversation impossible. Poor relationships between workers and management were noted in most industrial outbreaks, ranging from rigid supervisory styles to unclear organizational structure. Physical irritants like fumes, temperature swings, and bad lighting added to overall stress levels even when environmental testing confirmed they weren’t at harmful concentrations. The irritants weren’t causing illness directly, but they were keeping people in a state of heightened bodily awareness where psychogenic symptoms had fertile ground.
How Social Media Changed the Equation
Traditional MPI outbreaks required physical proximity. People needed to see, hear, and interact with affected individuals for emotional contagion to take hold. Social media dissolved that requirement. The TikTok tics phenomenon of 2021 demonstrated that emotional contagion can operate through screens, across continents, among people who have never met. Adolescents watching videos of creators with tic disorders began developing similar movements and vocalizations, sometimes within days of heavy viewing.
This represents a fundamental shift in the scale and speed at which psychogenic symptoms can spread. The same mirror mechanisms that operate in face-to-face interaction are engaged by video content, particularly content that is emotionally compelling and comes from creators the viewer identifies with. The algorithmic structure of social media platforms, which serves more of whatever a user engages with, can create an immersive environment that mimics the closed, high-stress conditions historically associated with MPI outbreaks.
What Stops an Outbreak
Resolving mass psychogenic illness typically requires addressing the underlying stressors rather than the symptoms. In workplace outbreaks, investigators have found that improving communication, reducing production pressure, and giving people more autonomy over their work conditions helps symptoms resolve. Separating affected individuals from each other and from the environment also tends to help, which parallels what happens in shared psychotic disorder: remove the person from the social context driving the belief or symptom, and recovery often follows.
Public communication matters enormously. Dismissing people’s symptoms as “all in their head” tends to make outbreaks worse, because it adds frustration and mistrust to an already stressed group. Acknowledging that the symptoms are real while calmly explaining that no environmental threat has been found strikes a more effective balance. The goal is to reduce the anxiety and hypervigilance that keep the feedback loop running without invalidating the experience of the people affected.