Nudge Wiki

Concept

Mass Hysteria (Herd Mentality)

Also known as: Herd Mentality Bias, Mass Psychogenic Illness

conformity social-proof psychology

An extreme, collective form of the same everyday tendency to follow the actions of others: an individual's behavior or physical symptoms are unconsciously adopted by those around them, triggering a domino effect through a group with no single organizing cause. Tali Sharot frames it as one end of a spectrum whose milder, constant form is simple herd mentality — walking into a foreign bar and copying how other patrons order, or joining a queue outside an unfamiliar museum simply because a queue exists.

Historical and modern mass-hysteria cases. A 1983 outbreak beginning with a single Palestinian schoolgirl in the West Bank eventually affected 943 Palestinian girls and Israeli soldiers with breathing difficulties and fainting, escalating into an international diplomatic incident before a year-long joint investigation concluded the cause was psychosomatic, not chemical warfare (see Palestinian Mass Hysteria Case (1983)). The medieval "dancing mania" saw groups across mainland Europe dance erratically, sometimes for weeks, until collapsing from exhaustion or injury — the worst recorded outbreak, in Strasbourg in 1518, involved around 400 people and multiple deaths from stroke and heart attack (see Strasbourg Dancing Mania Case (1518)). Convents between the 15th and 19th centuries — highly regimented, low-autonomy environments — repeatedly produced outbreaks of reported demonic possession, including one convent where dozens of nuns began meowing like cats (see Convent Demonic-Possession Mass Hysteria Cases). More recently, Havana Syndrome — a cluster of unexplained symptoms (strange sounds, sensations of heat or pressure) affecting US diplomats and intelligence officers from Cuba outward to China, Russia, and Vietnam since 2015 — is judged by multiple psychologists to be a modern instance of the same phenomenon, rather than a coordinated directed-energy attack (see Havana Syndrome Case).

The everyday mechanism has been directly measured. Researchers at Leeds University asked 200 volunteers to walk randomly around a hall with no talking, secretly instructing a small subset where specifically to walk. Result: it took only 5% of the room quietly walking with apparent purpose to determine the direction the other 95% ended up following, with no one aware they were being led (see Leeds University Confident-Walker Study). Tali Sharot and colleagues at the Weizmann Institute found the effect extends to memory itself: participants who watched a documentary and later saw other participants' (secretly falsified) answers about it conformed to the wrong answer 70% of the time, and half of them retained the false memory permanently even after being told the other answers had been fabricated (see Sharot Documentary Memory-Conformity Study). A separate Sharot study found 20% of hungry participants chose a food they'd explicitly said they disliked once told (falsely) that it was popular with others (see Sharot Fasting-Londoners Food-Conformity Study).

Real-world consequences of the same pull. A New York University study found that a restaurant's very first online review measurably shapes everything after it: an artificially glowing first review raised the odds of subsequent positive reviews by 32% and the restaurant's final average rating by 25% (see NYU Restaurant-Review First-Mover Conformity Study). Facebook's January 2012 mass emotional-contagion experiment, manipulating the news feeds of over half a million users to show more positive or more negative posts, found the manipulation measurably shifted what those users then posted themselves (see Facebook Emotional Contagion Experiment). The 2004 film Sideways, in which the main character viciously disparages Merlot while praising Pinot Noir, is credited with a real shift in the US wine market — Merlot's US market share fell from roughly 20% to 13% while Pinot Noir sales rose 16% over the same period (see Sideways Merlot-Pinot-Noir Wine Sales Case). Even baby names follow the pattern: the US name "Mason" jumped from 34th to 2nd most popular the year after Kourtney Kardashian's son was given the name, though Sharot cautions the celebrity may simply reflect a trend already underway rather than cause it.

Phill Agnew's own test. Inspired by this research, Agnew added two herd-mentality lines — "the UK's most popular marketing show" and "join the 300,000 smart people who tune into Nudge" — to a podcast-network audio ad that otherwise ran unchanged month to month. Ads on this network typically raised downloads about 19% week-on-week; this version raised downloads 33% (see Nudge Podcast Herd-Mentality Ad Test).

Herd mentality isn't unique to humans — Christophe Chamly's account of Antarctic penguins deciding whether to dive for krill (waiting for one penguin to jump first, then watching whether it's attacked before the rest follow) is offered as a non-human parallel to the same wait-and-copy strategy.

A literary illustration and a mundane one (Episode 280). Lord of the Flies' descent into violence is cited as a literary depiction of de-individuation — group identity overriding individual judgment and dissent. More mundanely, choosing to rewatch Home Alone every Christmas is partly herd mentality: everyone else does too.

Discussed in

  • Episode 165 — 165-is-mass-hysteria-more-common-than-you-think
  • Episode 280 — 280-when-you-can-t-stop-seeing-the-thing-you-ve-just-discovered (Lord of the Flies de-individuation; Home Alone rewatching habit)

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