Encyclopædia Britannica, Inc.

Lemmings are small creatures with wild reputations. In the 17th century, naturalists perplexed by the habit of Norway lemmings to suddenly appear in large numbers, seemingly out of nowhere, came to the conclusion that the animals were being spontaneously generated in the sky and then falling to earth like rain. (The prosaic truth is that they migrate in herds.) Some people also thought that lemmings explode if they become sufficiently angry. This is also a myth, of course—lemmings are indeed one of the more irascible rodents, but they mostly channel their rage into fights with other lemmings. People probably came up with the notion of exploding lemmings after seeing the picked-over lemming carcasses that were left behind following a migration.

But there is one myth that has held on tenaciously: Every few years, herds of lemmings commit mass suicide by jumping off seaside cliffs. Instinct, it is said, drives them to kill themselves whenever their population becomes unsustainably large.

Lemmings do not commit suicide. However, this particular myth is based on some actual lemming behaviors. Lemmings have large population booms every three or four years. When the concentration of lemmings becomes too high in one area, a large group will set out in search of a new home. Lemmings can swim, so if they reach a water obstacle, such as a river or lake, they may try to cross it. Inevitably, a few individuals drown. But it’s hardly suicide.

So why is the myth of mass lemming suicide so widely believed? For one, it provides an irresistible metaphor for human behavior. Someone who blindly follows a crowd—maybe even toward catastrophe—is called a lemming. Over the past century, the myth has been invoked to express modern anxieties about how individuality could be submerged and destroyed by mass phenomena, such as political movements or consumer culture.

But the biggest reason the myth endures? Deliberate fraud. For the 1958 Disney nature film White Wilderness, filmmakers eager for dramatic footage staged a lemming death plunge, pushing dozens of lemmings off a cliff while cameras were rolling. The images—shocking at the time for what they seemed to show about the cruelty of nature and shocking now for what they actually show about the cruelty of humans—convinced several generations of moviegoers that these little rodents do, in fact, possess a bizarre instinct to destroy themselves.