1894 Falling cats
In 1894, the Academy of Science in Paris requested “a physical clarification of how a cat is able to land on its feet after falling from a great height”. For nonscientists, the answer was easy: cats are simply so skilled at adjusting themselves in the air that they can position their feet underneath for landing. But people a little more in the know suspected that there might be some complicated physics involved.

The problem is that a falling cat has nothing to push against. Each turn that it makes with its forequarters causes its hindquarters to turn in the opposite direction. A half-clockwise turn in front means a half-counterclockwise turn behind. Theoretically, the cat should land all twisted up, which obviously is not the case.

At first, researchers assumed that the cat repelled off the hands of the experimenters. But even binding the cat’s paws individually with string to prevent it pushing before falling did not keep it from wriggling itself around right. A hypothesis that cats use the air for resistance also turned out not to be true.

The riddle was finally solved by the French doctor Étienne Jules Marey. Marey was a tinkerer who invented all sorts of mechanical devices, including a film camera that could capture a cat falling at 60 images a second. At a demonstration of the film, some physicists still doubted that the rotation was possible without the cat repelling in some way. But one physicist took a closer look at the pictures and realized the cat’s trick.

The movement occurs in two phases: First, the cat turns its forequarters toward the ground, then—in the same direction—its hindquarters. Changing the position of its paws between the two phases allows the cat’s front and rear to repel off each other. The cat uses the same principle as an ice skater executing a pirouette who pulls her arms in for fast spins and extends them to turn more slowly. The cat does both moves simultaneously: it pulls in its forepaws and thrusts out its hindpaws. That way it is able to quickly make a half-turn of its forequarters toward the ground, while its rear end turns only a little in the opposite direction owing to the resistance created by extending its hindpaws. To bring its hindquarters around, the cat reverses the procedure, thrusting out its forepaws and pulling in its hindpaws.

Marey’s film strips sparked a trend in filming falling animals. Soon people were dropping dogs, rabbits, monkeys and, in one study, a “fat little guinea pig”, who could twist its belly 180 degrees, to the amazement of the researchers. Researchers blindfolded dogs, and also tested animals without tails or organs of equilibrium. Even such a cat could rotate problem free. Apparently cats orient themselves mainly with their eyes.

In the 1960s, one researcher summed up 70 years of research on falling cats: “As can be seen, the turning cat raises a lot of interesting problems, even though their solution may not be of much practical importance—except to other cats.”





www.expo-marey.com offers a comprehensive online exhibit about Marey with details of his life, descriptions of his cameras, and many films, including films of falling cats, dogs, and rabbits (English and French)

Cat 1 (Film 00:03)
Cat 2 (Film 00:03

Dog (Film 00:03)

Rabbit 1 (Film 00:03)
Rabbit 2 (Film 00:03)
Rabbit 3 (Film 00:03)

Marey, E.-J. (1894), Mécanique animale: Des mouvements que certains animaux exécutent pour retomber sur leurs pieds lorsqu’ils sont précipités d’un lieu élevé. La Nature, S. 369–370. Fulltext


1902 Dr. Pavlov only rings once
The Russian physician Ivan Petrovitsch Pavlov holds an unusual record: no experiment has had more music groups named after it than the one Pavlov carried out with dogs at the beginning of the 20th century. A rock band called “Pavlov’s Dog and the Condition Reflex Soul Review and Concert Choir” formed in the 1970s, followed in the 1980s by “Ivan Pavlov and the Salivation Army”; in the 1990s the bluegrass band “Pavlov’s Dawgs” and the rock band “Conditioned Response” appeared, joined in the new millennium by the British folk group “Pavlov’s Cat.” Nor were musicians the only ones whose search for a name ended with “Pavlov.” “Pavlov’s Dog” is the name of a communications firm in Ireland, a pub in England, a theater group in Canada, and a drink at the One World Café in Baltimore, Maryland—a mix of Kahlua, Bailey’s, and milk.

In 1904, Pavlov won a Nobel prize for his research on digestion. But that isn’t why his name is so popular today. The bigger reason is the fundamental mechanism of learning he stumbled on while carrying out his work.

In his studies on digestion, Pavlov also became interested in how the salivary gland works. To be able to observe the salivary activity of living dogs, Pavlov funneled the saliva through holes in the animals’ cheek and into a small measuring cup. Actually, he wanted to see how the saliva formed when the dogs were fed a variety of foods. But soon a problem arose. After the dogs had been fed a few times, they began to salivate as soon as they saw the food. At first Pavlov regarded this effect as a nuisance and devised techniques for getting the food into the dogs’ mouths without warning. But it turned out that the animals also associated very subtle signals with the food. All it took was the sight of the researcher or the sound of his step to get the saliva flowing.

Very soon Pavlov saw this phenomenon no longer as a flaw in his experiment but as a new area of research. He did experiments in which he controlled the signals that occurred before the feeding: Five seconds before, a metronome would start sounding, or an electric bell. After such a coupling—with the bell, it took only once—the dogs salivated as soon as they heard the signal. They had learned that they would be fed after the sound of the bell. Because the dogs interpreted the least little indication from their environment as a signal for feeding, Pavlov had a new building built in St. Petersburg with soundproofed rooms, in which all the necessary manipulations could be carried out with remote levers and cables.

This fundamental learning mechanism that Pavlov discovered is called classical conditioning. It paired a natural stimulus-reaction response (food-salivation) with a new stimulus (bell). But though a new stimulus can activate innate behavior, in almost any combination, it does not give rise to genuinely new behaviors. How new behaviors are learned was taken up only 30 years after Pavlov by the Amerian psychologist B. F. Skinner with his so-called Skinnerbox.

In addition, Pavlov found out through his research how conditioning can be lost again: ring the he bell a few times without feeding the dog afterwards, and it unlearns the connection. This principle later became the basis for behavioral therapy, in which patients confront situations such as those that produce anxiety. In this way, the association between situation and anxiety is erased.

Today Pavlov is a household name. For cultural critics, his dogs are a symbol for the masses of people in western industrialized societies who let advertising transform them into consumer animals programmed to buy in response to specific stimuli.

Unlike Pavlov himself, one of the most famous scientists of all time, the bands that named themselves after him have never made a breakthrough, or at least, not yet. The closest any of them got was the rock group “Pavlov’s Dog and the Condition Reflex Soul Review and Concert Choir,” which in 1973 in changed its name to“Pavlov’s Dog.” For their debut album, they received $600,000, at the time the biggest advance ever paid in the United States for a record. Three years later the record company dropped the group, and the musicians went bankrupt and disbanded.




A short film (02:28) explains classical conditioning.

Animation (01:17) explaining classical conditioning.

Train a dog at the "Nobel e-Museum" to drool on command! Also: further information on Pavlov.

Cartoons about Pavlov's experiments.

No experiment has had more influence on the names of rock bands and the titles of songs:

“Pampered Menial” by
Pavlov's Dog.

“Pavlov’s Dog” by Conditioned Response.

“Lost in Space” by Aimee Man. One of the songs title is “Pavlov's Bell”.

Pavlov, I. P. (1927), Conditioned Reflexes: An Investigation of the Physiological Activity of the Cerebral Cortex. Oxford University Press. Fulltext



1907 The 21-gram soul
The story was so hot that even the New York Times carried it. “Soul has weight, physican thinks,” read a headline on page 5 of the newspaper on March 11, 1907. The article reported the curious experiment of a certain Duncan MacDougall, a doctor from Haverhill, Massachusetts.

MacDougall had long been working on the nature of the soul. According to his peculiar logic, if the psyche continues to function after death, it must occupy space in the living body. And because everything that occupies space also has weight, according to the “latest conception of science,” the soul could be detected “by weighing a human being in the act of death.” So MacDougall constructed a precision weighing machine: a bed arranged on a framework whose weight, with contents, could be ascertained to within exactly 5 grams.

The very sensitivity of the scales, however, greatly restricted the choice of research subjects. “It seemed to me best to select a patient dying with a disease that produces great exhaustion, the death occurring with little or no muscular movement, because in such a case the beam could be kept more perfectly at balance and any loss occurring readily noted,” wrote MacDougall later in the journal American Medicine. People dying of pneumonia, for example, were unsuitable. They would “struggle sufficiently to unbalance the scales.”

The best subjects proved to be patients with tuberculosis, whose last moments would be “as nearly inactive as could be found.” MacDougall found them at the Cullis Free Home for Consumptives. It is not known whether the patients or their families gave consent for the experiments. What is known is that some people were skeptical of MacDougall’s studies in biological theology. In the case of one of the research subjects who had been weighed, MacDougall complained that the “scales were not finely adjusted and there was a good deal of interference by people opposed to our work.”

MacDougall placed the first dying patient on his scales at 5:30 in the evening. Three hours and 40 minutes later, “He expired and suddenly coincident with death the beam end dropped with an audible strike hitting against the lower limiting bar and remaining there with no rebound.” MacDougal had to place two dollar-coin pieces on the scale to bring it back into balance. The difference was 21 grams.

The next five subjects painted a confusing picture: in two cases the measurements were unusable; a third patient’s weight decreased after death but remained stable after that; the weight of two others decreased and then went back up again; and the fifth patient’s weight sank, went back up, then sank again. In addition, MacDougall had difficulty specifying the exact time of death.

Yet such details did not deter him in his belief that he had proved the existence of the human soul. Indeed, he carried out a second experiment that confirmed his finding: 15 dogs (“between 15 and 75 pounds [6.8 and 34 kilograms]”) perished on the scales—all without the slightest loss of weight. MacDougall did not reveal in his article in American Medicine how he was able to persuade the dogs to die on his weighing machine, but in all likelihood he poisoned them. MacDougall has not happy with this experiment. Not because he found it reprehensible to kill 15 healthy dogs out of scientific curiosity, but because the results could not be compared directly with those of his research subjects. Ideally, the test should have been done on dogs that also were so diseased that they could not move, wrote MacDougall: “It was not my fortune to get dogs dying from such sickness.”

Opinions of scientists on MacDougall’s soul-weighing machine diverged widely. Some of his colleagues thought the experiments were silly; others felt MacDougall had made “the most important addition to science that the world has known” and discussed how his method could be improved. The use of dying subjects struck them as particularly troublesome because the rapid onset of decomposition could explain the change in weight. “How much more satisfactory it would be if the subjects were normal men in perfect health,” a New York doctor was quoted as saying in the Washington Post. He suggested hanging the electric chair on a scale and determining a condemned person’s weight before and after electrocution.

MacDougall conducted further experiments and attracted attention again in 1911 when he affirmed that he had observed the soul leaving the body, “a strong ray of pure light.”

The only enduring legacy of this experiment is the weight loss that occurred in the first research subject: for a hundred years the idea that the soul weighs 21 grams has persisted in popular culture. In 2003, that notion even made it into the movies. In a film titled “21 Grams,” director Alejandro González Iñárritu explored the deeper meaning of life and death.




(The Washington Post, 12. 3. 1907). Read the article

(The Washington Post, 13. 3. 1907). Read the article

(The Washington Post, 18. 3. 1907). Read the article

The title of the film "21 Grams" (2003) (with Sean Penn, Charlotte Gainsbourg)comes from the work of Dr. Duncan MacDougall. MacDougall weighed the soul. One of his results: 21 grams.

MacDougall, D. (1907/April), Hypothesis Concerning Soul Substance Together with Experimental Evidence of The Existence of Such Substance. American Medicine. Fulltext


1914 Stairway to a banana
The picture has great symbolic value: A chimpanzee stands on a pile of three wooden crates and grabs for a banana. Generations of psychology students have interpreted the scene as a clever monkey suddenly deciding to pile one crate on top of another to reach an otherwise unattainable fruit. Of course, the story is a little more complicated than that.

The German psychologist Wolfgang Köhler, who devised this intelligence test for monkeys, arrived in Tenerife at the end of 1913 to take over the direction of the Anthropoid Station there. He actually intended to stay only a year, but the First World War intervened, and one year turned into six.

During this time, Köhler conducted a series of elegant experiments on the intelligence of apes. In so doing, he became convinced that “chimpanzees manifest intelligent behavior of the general kind familiar in human beings.” This view reassured the then evolutionary biologist. Only a brief time had elapsed since Darwin established the theory of natural selection, and biologists were looking everywhere for clues to confirm it. The physical similarity of men and apes was one piece of circumstantial evidence pointing to their kinship, but Darwin was convinced that men and apes were also close intellectually. How close is what Köhler hoped to discover through his experiments.

On January 24, 1914, he led six of his chimpanzees into a two-meter-high room, hung a banana in the corner, and placed a wooden box in the middle of the floor. Then he waited. All the animals tried in vain to leap for the banana. “Sultan soon relinquished the attempt,” wrote Köhler, “paced restlessly up and down, suddenly stood still in front of the box, seized it, tipped it hastily straight towards the objective, but began to climb upon it at a (horizontal) distance of half a meter, and springing upwards with all his force, tore down the banana.” Sultan had solved the problem. He had unexpectedly and singlemindedly acted as though he had had a sudden insight.

Köhler was all the more surprised when his chimpanzees failed repeatedly at the next problem: The banana was hung higher, and the monkeys could only reach it by piling the boxes on top of one another. Köhler noted that the problem for the chimpanzee “falls into two very distinct parts: one of which he can settle with ease, whilst the other presents considerable difficulties.” The easy part was to shove a box under the banana, the hard part, to pile a second box on the first. This “curious fact” perplexed Köhler, because the difficulty for humans is totally different. When a person first realizes that he can get near the banana by putting a box under it and standing on the box, it then becomes clear that the greater height afforded by piling up two or three boxes will allow him to actually reach the fruit. For a human, “adding a second piece of building material to the first is only a repetition of the placing of the first one on the ground.” Not so for a monkey.

Grande, the chimpanzee in the picture, struggled harder and harder with the second box. Over time, she succeeded in building a little structure, but for years she kept making the same mistake. And even after many successful attempts in the right direction, she would suddenly be utterly clueless again about what to do with the second box. Köhler concluded that a chimpanzee has no insight at all into the mechanics of its constructions: “Almost everything arising as ‘questions of statics’ during building operations, he does not solve with insight, but by trying around blindly.”

Köhler’s experiments are considered classic today, and are still being carried out in modified form. But what they prove about resemblances between people and apes is hard to say. Similar behavior between people and chimpanzees does not necessarily imply similar ways of thinking.




The Web page accompanying the American PBS documentary series Inside the Animal Mind offers a short film (00:53) of one of Köhler’s experiments. The site also contains other video sequences on the themes of animal intelligence, consciousness, and emotion.

The first intelligence test for animals in 1914. (picture from Köhler, W. (1921), Intelligenzprüfungen an Menschenaffen, Springer).

Köhler, W. (1921), Intelligenzprüfungen an Menschenaffen. Springer.


1928 The living dog’s head
In the photographs it looks like a circus side show. In the middle sits a bowl containing a severed dog’s head and two tubes leading to a stand holding a pump, a bottle, and a basin filled to the brim with blood. A group of onlookers clusters tightly round about, witnesses to a scientific miracle: the dog’s head lives.

The Russian surgeons Sergei Brukhonenko and S. Tchetchuline had removed the dog’s head from its body during an operation that the popular science magazine Science and Invention described as “grisly and inhuman”—not failing, however, in the next sentence to point out the great utility of animal research. Now the dog’s head lay with its mouth half open, and it seemed as though the scientists were determined to show the audience in as many ways as possible that the head was really alive. They shined a flashlight in its eyes until its pupils narrowed, smeared honey in its mouth, which immediately lapped it up, made its eyes tear with quinine, and gave it sweets that tumbled out of the stump of its esophagus after it had swallowed them.

Brukhonenko and Tchetchuline were not the first to experiment with severed heads. But in earlier investigations, the head had been kept alive with a mechanical heart. Blood was ferried from the carotid arteries through rubber tubes into an open basin, where it was combined with oxygen, and from there into a bottle somewhat above the dog’s head, flowing under constant pressure back into the arteries. An electrical pump drove these primitive heart-lung machines. The blood was chemically treated beforehand to keep it from clotting.

This bizarre experiment stimulated people’s imaginations. Would it be possible with a human head, too? Is that what eternal life on Earth would look like? A French researcher suggested founding a society for the prevention of death, and enthusiastically asked Science and Invention, “Do not even the wildest imaginations of our modern science fiction writers pale into insignificance because of the steady advance and progress of scientific research?”




Not for the fainthearted: Russian surgeons keep a severed dog’s head alive. Film (19:31)

Living head in a Russian experiment.

Like a circus side show: presentation of the living dog's head.

Brukhonenko, S.S. and Tchetchuline, S. (1929) Expériences avec la tête isolée du chien. Journal de Physiologie et de Pathologie Générale 27 (1), 31-45, 64-79.


1928 The curve of ecstasy
The cardiotachometer developed by the American physician Ernst P. Boas was the dream of every heart specialist. It enabled automatic and continuous recording of cardiac activity while a research subject was physically active. Up to that point, all instruments had required subjects to remain immobile.

Boas and his colleague Ernst F. Goldschmidt set about measuring the life of 51 men and 52 women by taking their pulse. Doing that allowed them to determine the maximum heart rate during various activities: eating (102), talking on the phone (106), getting washed in the morning (106.7), listening to music (107.5), dancing (130.6), and exercising (142.6). But the winning rate was 148.5 beats per minute attained during orgasm. Boas and Goldschmidt’s book The Heart Rate doesn’t say much about exactly how this measurement was carried out. “We were fortunate in obtaining a record of the heart rates of a man and wife during intercourse,” they wrote, and then focused on describing the results. That orgasm taxes the heart more than exercise was not surprising, but the two physicians wrote nonchalantly about a second, quite remarkable feature of the heart diagram as if nothing could be more normal: “It shows four peaks of heart rate for the woman, each peak representing an orgasm.” On the night in question, between 11:25 and 11:45, the woman experienced four orgasms. Moreover, she did it despite two uncomfortable rubber electrodes attached to her chest that were connected to the measuring apparatus by a roughly 30-meter-long cable.

Boas and Goldschmidt kept their comments to a minimum: “The curve of heart rate clearly indicates the strain placed on the cardiovascular system, and helps to explain some cases of sudden death during and after coitus. Pussepp [another researcher] has demonstrated marked rises in blood pressure in dogs during coitus.”

The four peaks were not, however, lost on sex researcher Robert Latou Dickinson, who used the chart in his book Human Sex Anatomy. Still, he attributed the feat to the ability of the man, whose “developed technique” allowed him to remain in his wife’s vagina for 25 minutes “awaiting complete satisfaction on her part.”
The fact that women taking in experiments dealing with orgasm are not necessarily average was demonstrated 25 years later by a subject whose metabolism was investigated during intercourse.




Recording of cardiac activity during intercourse. Between 11.20 PM und 11.45 PM the woman experienced four orgasms! One researcher attributed the feat to the “developed technique" of the man. (graph from: Dickinson, R.L. (1949), Human Sex Anatomy. Williams & Wilkins).

Boas, E. P., and Goldschmidt, E. F. (1932), The Heart Rate. C. C. Thomas.


1930 Mr. Skinner’s box
Burrhus Frederic Skinner could have had no idea that the box that he put together in the laboratory of the psychology department at Harvard University would become one of the most famous pieces of apparatus ever constructed for an experiment. Later, a rock band would be named for it. Cartoonists depicted it in their drawings, and it was parodied in the animated film series The Simpsons. The cage with the automatic feeder was even said to be behind the rumored suicide of Skinner’s daughter.

Skinner was 26 years old when he went looking for an instrument to measure the behavior of rats. The maze, popular at the time, seemed to him less than ideal. “The animals’ behavior was composed of too many different ‘reflexes’ and should be taken apart for analysis,” he wrote in his memoirs. So he focused instead on a small section of the experimental setup: a soundproof box with a noiseless door through which a rat could be let undisturbed into a maze. But Skinner soon dispensed with the maze. He attempted to record the movements of animals using exact measuring devices. But the recordings were too chaotic to interpret. Skinner read Pavlov, who 30 years earlier had discovered classical conditioning, in which innate reactions can be coupled to new stimuli. But Skinner didn’t intend only to investigate existing reactions. He wanted to understand how new behavior comes about.

Eventually it occurred to him to equip his experimental box with a lever. Whenever the rat pressed the lever, it received a food pellet. Naturally, at first the rat didn’t know that, and managed to trigger pellets by unintentionally touching the lever. But following a few such strokes of luck, the rat appeared to have learned the association, and the time between two pressings of the lever became increasingly shorter. Skinner had discovered a simple way of measuring behavior change in a rat, namely, how often the behavior occurs.

Unlike Pavlov’s experiment, the animal showed no innate reaction, but learned a new behavior. The theory that Skinner constructed consisted of three parts: animals constantly show spontaneous behavior; the consequences—positive or negative—of a behavior increase or decrease the likelihood that an organism will behave that way again; and these consequences are determined by the environment. Skinner called the entire process “operant conditioning” (as opposed to Pavlov’s classical conditioning).

What was happening in the brain did not interest Skinner. Because there was no way to directly observe the workings of the mind, he did not think it could be tackled scientifically. Instead, together with John B. Watson, Skinner led the school of behaviorism, which describes the behavior of humans and animals as the consequence of reactions to stimuli.

Compared with earlier instruments, such as the maze, the Skinnerbox had a major advantage: after the rat had pressed the lever and received the food pellet, everything was ready for the animal’s next action without human intervention. An automatic writer recorded when the lever was pressed, and Skinner used these data to study the learning behavior under various conditions. What happened when the rat had to press the lever five times in a row to get the food pellet, or when it was rewarded only after a random number of presses? What happened when the animal found a way to avoid being penalized? How could a learned behavior be unlearned? The Skinnerbox was a kind of automated animal research.

The methods of operant conditioning seem mundane—rewarding behavior strengthens it, and punishing it diminishes it. But Skinner taught animals to do a lot more than just press a lever. He taught a pigeon a melody on a toy piano, and two pigeons a version of table tennis. The trick there was not to wait until the animal had achieved the goal to reward it, but to reward it for each step along the way. For example, the pigeon received grains for making the first sound by accidentally pressing its beak on the toy piano. It received more grains when it correctly made the second sound, then the third, until it could play the child’s song “Over the Fence Is Out, Boys.”
With such training, humans could exploit the animals’ senses for all sorts of tasks. During the Second World War, Skinner worked for the American military on a curious bomb-steering system for attacking ships: pigeons were conditioned to serve as a primitive steering system in the nose of projectiles. Depending on the position of the ship, which the birds could see through a window at the end of the bomb, they pecked with their beak at various spots on a screen. These signals were used to steer the bomb. The mechanism worked in the laboratory but was never deployed.

Skinner did not himself coin the term “Skinnerbox,” but the name rapidly became popular. Skinner was even suspected of having raised his daughter Deborah in a Skinnerbox. Later, a rumor circulated that Deborah had ended up in a psychiatric institution and had committed suicide.

This myth originated with an article in the Ladies’ Home Journal in October 1945. The women’s magazine described the heated, soundproof crib that Skinner had built for Deborah. Unfortunately, the title of the article—“Baby in a Box”—led many readers to conclude that Deborah had been plunked in a Skinnerbox and subjected to experiments, like her father’s rats and pigeons. Today, Skinner’s daughter, who lives in London as an artist, turns up every now and again in the press to demonstrate that she is alive and well.

Skinner was a controversial figure in American intellectual life. His insights on upbringing were especially influential: the parallels between praise and blame and his experiments were obvious. For Skinner, the world was one big Skinnerbox. He was convinced that it could explain the entire human behavioral repertoire. In his controversial book Beyond Freedom and Dignity, published in 1971, Skinner suggested that conditioning techniques could be applied for the good of humankind to train people to behave in socially desirable ways.




B. F. Skinner on the cover of Time magazine, 1973.

Four tongue-in-cheek cartoon films about the Skinnerbox.

Even if you don’t want to take part in the Annual Skinnerbox Rat Training Competition, check out what Ruby the Rat was able to learn in six weeks through operant conditioning. (Film)

Cartoonist Craig Swansons interpretation of operant conditioning.

Listen to Skinner’s personal description of operant conditioning on the Website of the Society for the Experimental Analysis of Behavior. Audio

Skinner, B. F. (1938), The Behavior of Organisms: An Experimental Analysis, Appleton-Century.


1946 Vacation in the draft
Shortly after the Second World War, anyone in England wanting to take a cheap vacation traveled to Salisbury. At that time, in this small town situated roughly 150 kilometers southwest of London, two could live cost-free in spacious apartments outfitted with books, games, radio and telephone, and could pass the time playing table tennis, badminton, or golf—and even get paid 3 shillings a day to do it.

There was only one catch: The buildings of Harvard Hospital, set on a windy hill a bit out of the way, housed the British government’s department for cold research—the Common Cold Unit—and the visitors, mostly students, served as research guinea pigs. “Unsatisfactory” indeed, as the head of the Common Cold Unit, Christopher Howard Andrewes, wrote in an article in 1949, but “the only animals available.”

At the time, aside from humans only chimpanzees could transmit a cold. And according to Andrewes, they were “very expensive, strong, and difficult to handle.” Students were entirely different: the “ten days’ free holiday” in the Harvard Hospital was popular. Some of the research subjects returned there several times.

The 12 people who had to wait in a drafty passageway for half an hour on a Saturday morning following a bath in hot water may not have been among them. They felt “chilly and miserable,” wrote Andrewes, and their mood was probably not helped by the wet socks they had to wear for the rest of the morning.

If you believe old wives’ tales, then this treatment is a recipe for catching a nasty cold. And it was exactly this belief that Andrewes wanted to test scientifically, because some observations contradicted it. Arctic researchers who took off on long expeditions never caught cold. And in Eskimo villages, people did not get sick in winter, when it was coldest, but in spring, after the first ships carrying outsiders sailed into harbor.

The research subjects with the wet socks had arrived in Salisbury three days before. Like all the previous experiments in the Common Cold Unit, this one, too, began on a Wednesday. The subjects submitted to an initial examination, and were assigned in pairs to the 12 apartments. They were also instructed during the next 10 days to keep at least 10 meters away from all unprotected people except for their roommates. Although walks were allowed, buildings and transportation were to be avoided. Doctors and nurses wore masks and protective clothing while examining the subjects. Three times a day, thermos containers of meals were left at the doors to the apartments.

The days between Wednesday and Saturday elapsed without any notable activity. The purpose of waiting was to be able to detect any colds that may have started before the actual experiment.

On Saturday morning, the doctors divided the research subjects into three groups of six. The first six had filtered and diluted nasal secretions from a person with a cold dripped into their nostrils. The next six underwent treatment with cold baths, drafts, and wet socks. The final group received both cold treatment and nasal secretions.

At the time, it was firmly believed that colds were caused by viruses primarily found in the nasal secretions. Because the major symptom is a badly stuffed-up nose, in Salisbury the daily increase in the weight of handkerchiefs was taken to be an index of the seriousness of the infection. To be able to precisely determine the pathogen, it would have had to be cultured, which proved difficult.

A few days following Saturday’s treatment, the first participants got sick, namely, four of the research subjects who had received both viruses and cold treatment and two who had only been infected with the virus. Freezing alone produced no cold.

The popular belief seemed to be confirmed: although by itself exposure to cold didn’t make people sick, it apparently favored viral activity. But Andrewes wasn’t satisfied with that, because the number of research subjects was too small to draw any definite conclusions. “We were foolish enough to repeat this experiment, with a contrary result.”

Once again, chill alone caused no colds, but in the group only infected with the virus, twice as many people became sick as in the group that had stood in the cold in addition. A third experiment produced the same result: there was still no connection between a cold and previous exposure to chill.

Andrewes’s experiments were the first of a whole series carried out with hundreds of research subjects during the 1950s and 1960s. Not a single one showed that being cold has anything whatsoever to do with catching cold.

Why colds occur more often in winter than in summer in our latitudes is still not entirely clear. Further studies have shown that apparently neither a weakened immune system nor the dry air in heated rooms has any bearing on this greater frequency. It is more likely that in winter, the virus is more easily transmitted as people gather together in badly ventilated rooms. Even sunlight, whose ultraviolet rays kill germs, is not as strong in winter.

But Andrewes had already recognized that science was at a disadvantage compared with popular belief. “Even the most eminent men of science almost invariably lose all sense of critical judgment when colds and especially their own colds are concerned.”

Especially when the cause of the ailment and what we call it are so closely identified.




An entertaining film clip (02:30) from 1947 about the common cold experiments in Salisbury.

Further information is available on the Web site of leading cold researchers Jack M. Gwaltney and Frederick G. Hayden. The list of common cold myths is especially worth reading.

Andrewes, C. H. (1948), Cantor Lecture: The Common Cold. Journal of the Royal Society of Arts 103, S. 200–210.


1961 Obedient to the end
When Morris Braverman entered Yale University’s Linsly-Chittenden Hall in New Haven, Connecticut, in the summer of 1961, he could not have known that one hour later he would have tortured another person for no reason. Braverman, a 39-year-old social worker, answered an ad in a local newspaper: “We will pay five hundred New Haven men to help us complete a scientific study of memory and learning.” The compensation for “approximately one hour’s time” was $4.00 plus 50 cents for carfare. Braverman mailed his application to the appropriate address. A few days later, he was summoned by telephone.

What happened next turned out to be an extremely controversial study in social psychology. Some consider it the most important experiment on human behavior ever carried out; others feel it ought never to have taken place. It was soon known simply as the “Milgram experiment”, after Stanley Milgram, the 27-year-old assistant professor who thought it up. With time it became so familiar that it turned up in newspaper reports of the genocide in Rwanda and in the TV series “The Simpsons”. A French punk rock band calls itself Milgram, and a comic team in New York is known as the Stanley Milgram Experiment. Indeed, Milgram’s experiment became world famous; it also cost him his career.

On entering the laboratory, Braverman was greeted by the experimenter, a young man in a grey lab coat who introduced him to another research subject who had already arrived: James McDonough, a 47-year-old accountant from West Haven. First, the experimenter explained to the two men that the goal of the experiment was to measure the effects of punishment on learning. Thus one would take the role of “teacher”, the other the role of “learner”. The experimenter next had Braverman and McDonough draw labeled slips of paper to determine their role. What Braverman didn’t know was that the drawing was rigged: each slip of paper was marked “teacher”. McDonough was an actor who was only pretending to be a research subject. For Milgram’s experiment it was essential that a naive subject, in this case, Braverman, play the teacher.

After the drawing, Milgram took McDonough into an adjacent room where he bound him to a chair that looked a little like an electric chair. Milgram tied an electrode to McDonough’s left wrist, explaining to Braverman that it was connected to a shock generator in the control room. McDonough’s right hand had just enough freedom that his fingers could reach a four-lever switchboard placed on a table. In response to a question from McDonough about the strength of the electric shocks, Milgram answered that they were “extremely painful”, but that there was no fear of “permanent tissue damage”. Back in the control room, he explained Braverman’s task to him. Over a two-way intercom to McDonough in the next room, he was to read out pairs of words: “blue—box”, “nice—day”, “wild—bird” and so on. In a second round, Braverman was only to provide the first word of each pair. McDonough’s task was to remember the second word of each pair. So when Braverman said “blue” and then gave McDonough four possibilities—“day”, “box”, “sky”, “bird”—McDonough had to choose the correct response using the switchboard.

When McDonough pressed the correct lever, Braverman was to go on to the next word in the list. On the other hand, if McDonough answered incorrectly, Braverman was to punish him with a jolt of electricity—15 volts for the first error, 30 for the second, 45 for the third and so on, up to a maximum of 450 volts. Braverman had before him for the task an instrument equipped with a long row of lever switches and labeled “Shock Generator, Type ZLB, Dyson Instrument Company, Waltham, Mass., Output 15 Volts-450 Volts”. Had Braverman been familiar with the city of Waltham, he would have known that no such company existed.

Milgram conceived the idea for his experiment in 1960, while he was a student at Princeton University in New Jersey. Through another experiment that later became widely known, Milgram’s mentor at Princeton, the psychologist Solomon Asch, demonstrated the substantial pressure a group can exert on a single person. In a matching test research subjects deliberately made false conclusions to comply with the majority.

Milgram wished to test the influence of group pressure in a less benign situation. Could a research subject be led to cause another person pain for no good reason? Milgram used pilot tests to determine how far subjects would go without group pressure. These tests showed that the group wasn’t necessary at all: a single person was enough.

Braverman knew nothing of this when, following McDonough’s first error, he administered a 15-volt electric shock. McDonough made further errors, and as he had been instructed at the outset, Braverman raised the voltage, 15 volts each time.

Following the 120-volt shock, McDonough told the experimenter over the intercom that the shocks were becoming painful. At 150 volts McDonough cried out, “Experimenter, get me out of here! I won’t be in the experiment any more! I refuse to go on!” At 180 volts: “I can’t stand the pain.” At 270 volts McDonough screamed and said he would give no further answers.

Braverman turned to the experimenter, who said, “Please go on,” and instructed him to treat no answer as a wrong one and to punish the student with a shock. Braverman fidgeted in the chair and began to laugh nervously, but continued. McDonough now provided no answers but shrieked at each shock. Braverman turned again to the experimenter: “Do I have to follow these instructions literally?” The experimenter said, “The experiment requires that you continue.” Braverman continued. At 330 volts, McDonough fell silent. Braverman offered to exchange places with him. But still he continued. The 375-volt toggle switch was labeled, “Danger: Severe Shock.” Braverman carried on up to the final toggle switch at 450 volts.

Morris Braverman, social worker from New Haven, was not the only person to administer electric shocks in the summer of 1961, even though the command to do so came from an experimenter totally lacking in authority. Workman Jack Washington, welder Bruno Batta, nurse Karen Dontz and housewife Elinor Rosenblum all continued right up to the end of the scale. Over a thousand researchers took part in various versions of Milgram’s experiment. Two-thirds administered shocks as high as 450 volts.
Milgram never anticipated such a result. No one did. In preliminary lectures, he described the experiment in detail and asked for responses from the audience. Neither psychologists nor laypeople came even close to predicting how readily people would obey. Most assumed that no one would go beyond 150 volts.

Milgram knew that his results were dramatic, but from a scientific standpoint the experiment posed a dilemma: it neither solved a problem nor confirmed a theory. Two scientific journals refused to publish it. Only on a third try, in 1963, after Milgram had described and compared various versions of the experiment, did his “Behavioral Study of Obedience” appear in the Journal of Abnormal and Social Psychology.

Milgram carried out nearly 20 variations of the experiment. Sometimes the learners complained of heart problems, sometimes the setting was dismal buildings far from the university, sometimes women administered the electric shocks. The results were always the same: more than half the research participants administered up to the maximum shock.

In other versions of the experiment, the learner was placed in the same room as the subject. Obedience sharply decreased; yet even when the experimenter commanded the subject to forcibly press the learner’s hand down on the electrode, still a third persisted in delivering up to 450 volts. Although physical nearness to the victim seemed to be important, more important was the nearness of the experimenter. When he gave his instructions over the telephone, only one out of five subjects obeyed.

Publication of Milgram’s experiment caused an immediate sensation. Newspapers not only reported the results but tried to interpret them. The main question—still unresolved today—was, Do ordinary people behave the same way as research subjects under pressure? Milgram himself always related the experiment to Nazi criminal behavior during the Second World War. Since the war’s end, the world had been seeking a rationale for the Holocaust. Milgram was convinced that human eagerness to obey might be one possible explanation.

When the study was published, the philosopher Hannah Arendt had just finished reporting on the trial of Nazi war criminal Adolf Eichmann in Jerusalem. In her now-famous article for the New Yorker magazine, she introduced the idea of the “banality of evil”. Arendt claimed that Eichmann was not the sadistic monster the prosecutor tried to present him as, but an uninspired bureaucrat who had simply been doing his job.

That was precisely in line with Milgram’s experiment. His research subjects were neither especially aggressive nor did they take any satisfaction in administering electric shocks to the learners. Quite the opposite: many were nervous, and began to sweat or to argue with the experimenter; but only a few had the presence of mind to quit the experiment. Refusing to obey is obviously so radical an act for people that they prefer to abandon their basic moral convictions. “The key of the behavior of subjects lies not in pent-up anger or aggression but in the nature of their relationship to authority,” concluded Milgram.

In September 1961, shortly after the startling results emerged, Milgram wrote to a funding agency, the National Science Foundation, “I once wondered whether in all of the United States a vicious government could find enough moral imbeciles to meet the personnel requirements of a national system of death camps, of the sort that were maintained in Germany. I am now beginning to think that the full complement could be recruited in New Haven.”

The association of the experiment with the Holocaust made Milgram a controversial figure, but more damaging was the accusation that his experiment was unethical. The question was how much stress research subjects should be exposed to. Some of Milgram’s colleagues felt Milgram had gone too far. He had expected the criticism, but was disappointed that no one seemed to appreciate the care with which he had constructed the experiment.

At the end of the one-hour experiment, the learner was fetched from the adjacent room, and told that he had actually received no electric shocks at all. In a follow-up session, Milgram queried all the subjects regarding their feelings about having participated in the experiment. Fewer than 2% regretted having taken part. Still, it would be hard to conduct the same experiment today. As a consequence of the stir over Milgram’s study, universities have set up ethical guidelines for approving research.

Only a few of the original participants are willing or able to talk about the experiment. Those still living among the over 1000 research subjects prefer not to discuss it. Milgram’s data lie anonymous in file boxes in the Yale University library. All the names of the research participants that surface in relation to the experiment have been changed, including the ones in this article.

One of the few contemporary witnesses is Milgram’s research assistant, Alan Elms. Today he is a professor of psychology at the University of California and recounts that many people always react with a mixture of fascination and disgust when they hear that he was involved with the experiment.

Milgram paid a high price for having delivered unsettling news about human nature. At Harvard University, where he was later assistant professor, he never received tenure. In 1967 he moved to the less hallowed City University of New York, where in 1984 he died of heart disease at the age of 51. His wife recently became a grandmother for the first time. She told a reporter that her grandson’s middle name was Stanley. Why not his first name? “I think it would be a burden to go around with the name Stanley Milgram," she answered.




A student film re-enacts the progress of the experiment using actors.

A French punk rock group named Milgram titled its CD "Vierhundertfünfzig Volt" (German for "450 volts").

Peter Gabriel wrote a song about the experiment: »We Do What We're Told (milgram's 37)«.

Stanley Milgrams biography: The Man Who Shocked the World: The Life and Legacy of Stanley Milgram (2004) Basic Books.

Milgram, S. (1963), Behavioral Study of Obedience. Journal of Abnormal and Social Psychology 67 (4), S. 371–378.


1967 Six degrees of separation
The question is one that has occupied mathematicians for a long time: A person chooses, at random, two other people in two places anywhere in the world. How many friends, friends of friends, and friends of friends of friends, on average, link the two people? In short: How many connections does it take two randomly chosen people on the planet to know each other? How small is the world?

At first glance, the solution to the so-called small world problem seems easy: when you know how many people a single person knows on average, you can make a quick calculation. For example, if I know 10 people, and if each of them knows another 10 people, then through two intermediary acquaintances I am already linked with 10 times 10 people, that is, 100 people, via three intermediaries with 100, via four with 10,000 and so on.

But the pair of mathematicians who did these calculations in the 1950s, Ithiel de Solla Pool of the Massachusetts Institute of Technology and Manfred Kochen of IBM, ran into two problems. The first seemed solvable: no data existed about the average number of people any single person knows. So they asked several people to keep a record of their contacts for a hundred days. Each knew, on average, 500 people. The second problem, however, had no solution: It is very likely that many friends of my friends know each other directly. Owing to these friends in common, using the above example I won’t reach 10 times more people with each intermediary, but clearly fewer. How many fewer depends on the closeness of the group in which I and my friends and their friends move, and also how closely those more distant groups are linked with each other. With an average of 500 acquaintances, the issue becomes so complicated after several intermediaries that de Solla Pool and Kochen decided not to publish the work they wrote up in 1958. “We never felt we had ‘broken the back of the problem,’” they wrote later. Yet their preliminary results did indeed suggest that people are linked by only a few intermediaries.

When the psychologist Stanley Milgram found out about their results, he set about testing them. Milgram’s experiment was later so popular, that it developed into a parlor game, and a play was named after the findings.

To begin with, Milgram chose a “target person”: the wife of a theology student at Harvard University, in Cambridge, Massachusetts, where Milgram was working at the time. His starting point was a few dozen people from Wichita, Kansas, or Omaha, Nebraska. They received the name of the target person with a short description as well as some instructions: “If you do not know the target person on a personal basis, do not try to contact him directly. Instead mail this folder...to a personal acquantance who is more likely than you to know the target person...it must be someone you know on a first-name basis.”

The first letter reached the target person after four days. It had begun its trip with a farmer in Kansas, who sent it to the minister in his home town. The minister sent it to a colleague in Cambridge who knew the wife of the theology student. The letter required only two intermediaries to reach its target. It was one of the shortest acquaintance chains that Milgram had ever observed. Surprisingly, in publishing his first study, Milgram described only this one result. The average number of intermediaries for his second experiment was 5.5.

The “small-world” idea spread like wildfire in the popular culture. In 1990, the American writer John Guare published his play Six Degrees of Separation, which alluded indirectly to Milgram’s experiment and was later turned into a movie with Will Smith in the title role. In 1994, three students from Albright College in Pennsylvania invented the game “Six Degrees of Kevin Bacon,” which turns on the idea of a random film actor who is linked to Kevin Bacon by the least number of films. For example, Will Smith acted in Welcome to Hollywood (2000) with Laurence Fishburne, who acted with Kevin Bacon in Mystic River (2003). That gives Smith a “Bacon value” of 2.

People are fascinated to think that every paddy farmer in China is two intermediaries away from Madonna. In the Czech Republic, there is even a heavy-metal band called “Six Degrees of Separation.” But although recently the mathematics has progressed somewhat, and people from all imaginable disciplines—from computer network specialists to epidemiologists—are interested in the “small world” phenomenon, the problem cannot be said to have been solved.

To date it is not clear whether Milgram’s six links are correct. He published his experiments not in a scientific journal, which is the usual practice, but in the popular science magazine Psychology Today. The data in the article are sketchy and difficult to confirm. For example, Milgram cited his success with the farmer in Kansas, whose letter managed to take just two intermediaries to get to Cambridge, but precise details of this study are tucked away in unpublished archival material. Only 3 of the 60 envelopes mailed from Kansas actually made it to the target—by way of eight intermediaries on average. The 5.5 intermediaries also turned up in later experiments of Milgram, who died in 1984. But sometimes the initial contact had deliberately sought out very gregarious people.

In 2003, scientists from Columbia University in New York repeated Milgram’s experiment with E-mails instead of letters. As targets, they selected 18 people from 13 countries. Just as in Milgram’s case, only a fraction of the chains were completed (384 out of 24,163). The average length of a chain from initial contact to target was 4.05. But this value is deceptive, because many of the chains never reached the target person. When that is taken into account, the value comes to between 5 and 7—surprisingly close to Milgram’s 6 intermediaries, but nonetheless not a clear-cut confirmation. After all, the participants in this experiment were hardly average citizens of the world: anyone without Internet access was automatically excluded.




John Guares play “Six Degrees of Separation” (1990) made the phrase “six degrees of separation” famous.

The film “Six Degrees of Separation” (with Will Smith) premiered in 1993.

You can play “Six degrees of Kevin Bacon” at oracleofbacon.org

Everything you wanted to know about the theory of networks in “Six Degrees” by Duncan J. Watts.

Take part in the Columbia University Experiment. At smallworld.columbia.edu you can find a target person to contact through as few E-mail intermediaries as possible.

Milgram, S. (1967), The Small World Problem. Psychology Today 1 (1), S. 60–67.


1971 The professor’s prison
In March 2001, American university professor Philip Zimbardo received hundreds of E-mails from Germany. “How could you do that?” asked the E-mail writers, who had just seen the film The Experiment. In the film, a psychologist puts 20 students in a simulated jail situation—10 in the role of the prisoners, 10 as their keepers. After three days, the situation gets out of control. The guards bind and strike the prisoners. Rapes and murders are committed. The story was inspired by an experiment that took place at Stanford University. The investigator was Philip Zimbardo.

In spring 1971, the then 38-year-old Zimbardo placed an announcement in the Palo Alto Times: “Male college students needed for psychological study of prison life. $15 per day for 1-2 weeks beginning Aug. 14. For further information & applications, come to Room 248, Jordan Hall, Stanford U.”

The idea for the experiment originated in Zimbardo’s course at Stanford. A few students chose the theme “psychology of imprisonment” and played jail for a weekend. Zimbardo was surprised by the profound impression the brief experience had on the students, and decided to probe the issue further.

From among the over 70 applicants who showed up in Room 248, Zimbardo selected the most normal and divided the group by coin toss into prisoners and jailors. Eleven students were notified by telephone that they would play prisoners, and should be ready at home on Sunday, August 15. Ten students played the role of guards, and were introduced to “prison director” Philip Zimbardo and his second-in-command David Jaffe—a research assistant—the day before the start of the experiment. The students were shown the jail, which had been set up in the basement of the psychology building. The cells were three small laboratories, whose doors were replaced with bars. There were surveillance rooms for the guards and a nine-meter-long corridor that would be used as an eating and exercise yard and was monitored by camera. An intercom system in the cells enabled the guards to give orders to the prisoners and to listen in on their conversations.

The guards picked out their uniforms together in a military-style clothing shop—khaki shirts and pants—and were given whistles, dark reflective glasses, and billy clubs. They were to work in eight-hour shifts and received a general instruction to “maintain the reasonable degree of order within the prison necessary for its effective functioning.”

The next day, the Stanford campus police arrested the 11 other students for breaking and entering. The police pulled up to their homes with sirens wailing, and took them away in handcuffs under the curious eyes of neighbors. The students arrived at the prison blindfolded, were forced to strip naked, and were photographed, deloused, and given their prison clothes: a sort of white smock with a number on the front and back (under which no underwear was permitted), plastic sandals, and a nylon stocking cap. The prisoners also had to wear a padlocked chain on one ankle.

During the short period of the simulation, Zimbardo tried to arouse in his prisoners the same feelings that real prisoners experience after a longer time: powerlessness, dependence, and hopelessness. The aim of the clothes was to humiliate the prisoners and to deprive them of their individuality. The shackle on the foot was to remind them where they were even in their sleep.

On the first day, the guards read aloud 17 rules that they had worked out with David Jaffe: “Rule Number One: Prisoners must remain silent during rest periods, after lights are out, during meals and whenever they are outside the prison yard. Two: Prisoners must eat at mealtimes and only at mealtimes....Seven: Prisoners must address each other by their ID number only....Sixteen: Failure to obey any of the above rules may result in punishment.” Repeatedly during each shift—as well as in the middle of the night—the guards called the prisoners to a head count, where they had to recite their ID number and the 16 rules. In the beginning, these inspections lasted 10 minutes. Later, they could take up to an hour.

Interestingly, Zimbardo had no idea what would happen in such a situation. The somewhat vague goal of the experiment was to find out “what the psychic effects of being a prisoner or an enforcement officer are.” He wanted to understand how prison inmates lose their freedom, independence, and privacy, while guards gain power by controlling the life of the inmates. His earlier experiments had shown how easily ordinary people can be led to behave in evil ways when they perceive themselves as part of a group rather than as individuals, or when they are led into a situation where they see other people as enemies or objects. The so-called Stanford Prison Experiment combined several of these mechanisms. It became so well known that a rock group in Los Angeles is named for it.

On the second day—after a head count at 2:30 in the morning—the prisoners rebelled. They took off their stocking caps, ripped the numbers from their smocks, and barricaded themselves in their cells. The guards forced them away from their doors with a fire extinguisher, and punished them. The ringleaders were thrown into solitary confinement, a dark closet at the end of the hallway. Anyone who hadn’t taken part in the rebellion enjoyed preferential treatment in a special cell and got better meals, Shortly after, without explanation, the guards put people from both groups in the same cell. This confused the prisoners, and they began to distrust one another. From that point on, they no longer protested as a group.

Now the guards began to impose absurd rules, disciplined the prisoners arbitrarily, and gave them pointless duties. They had to carry boxes from one room to another, clean the toilets with their bare hands, pick thorns out of their blankets for hours at a time (the guards had earlier dragged the blankets over thornbushes). And they were ordered to ridicule the rebels or to simulate sex acts with them.

After less than 36 hours, Zimbardo had to let prisoner 8612 go owing to extreme depression, uncontrolled weeping, and outbursts of anger. He hesitated at first because he believed that the student was only pretending to be at the end of his rope. Zimbardo could not imagine that a research subject in a simulated prison would exhibit such an extreme reaction after so short a time. But over the next three days, the same thing happened with three other participants. The subjects mistakenly believed that they could not quit the experiment.

Little by little, the border between experiment and reality began to blur for both the prisoners and the guards. The longer the experiment went on, the more often the guards had to be reminded that no physical force was allowed. The power that the experiment gave them turned peacefully inclined students into sadistic prison guards. Even Zimbardo behaved strangely. One day, one of the guards thought he overheard plans for a jailbreak being made. “How do you think we reacted to this rumor?” wrote Zimbardo later. “Do you think we recorded the pattern of rumor transmission and prepared to observe the impending escape? That was what we should have done, of course, if we were acting like experimental social psychologists. Instead, we reacted with concern over the security of our prison.” Zimbardo went to the Palo Alto police and asked to have the prisoners transferred to the city jail. When the police refused, he became angry and complained about the lack of cooperation between the prisons. Zimbardo himself had become a prison director. The planned breakout, of course, never happened. It had only been a rumor.

Next, Zimbardo worried that the parents of the students might demand to take their sons home after visiting hours. So he had the prison scrubbed from top to bottom, and the inmates were given good food and allowed to shower and shave. The visitors were welcomed by a young, attractive woman. They had to register and wait half an hour for a 10-minute visit. Several of the parents were shocked by the state of the prison, but they too seemed to accept the jail as reality and asked the director individually for better prison conditions for their sons.

A short time later, Zimbardo allowed in a Catholic priest who also had experience working in prisons. Half of the inmates introduced themselves to him by ID number. Without being asked to, he also played the role of a prison chaplain. Although the inmates had committed no crime, and Zimbardo had no legal power over them, the priest counseled them to consult a lawyer about getting out, just as he would have done in an actual jail.

On the fourth day, Zimbardo established a parole board composed of departmental secretaries and graduate students, to which prisoners could apply for early release. Almost everyone was prepared to forfeit their $15 per day in order to get out. The parole board returned the prisoners to their cells while they considered their applications. Amazingly, all the prisoners obeyed, although they could simply have quit the experiment by forfeiting the money anyway. But they lacked the will to do it. “Their sense of reality had shifted,” wrote Zimbardo, “and they no longer perceived their imprisonment as an experiment. In the psychological prison we had created, only the correctional staff had the power to grant paroles.”

In the meanwhile, a lawyer appeared who had been contacted by the parents of one of the students to get their son out. He discussed with the prisoner how to find bail money, and promised to return over the weekend—although he, too, knew that he was dealing with an experiment and that the question of bail was idiotic. At this point none of the participants had a clear idea where their role stopped and their own identity began.

Five days after the start of the experiment, on Thursday evening, Zimbardo’s friend and future wife, Christina Maslach, visited the prison. She was a psychologist and had already agreed to interview the inmates the next day. Nothing much was going on, and Maslach leafed through an article in the control room. Around 11 o’clock at night Zimbardo tapped her on the shoulder and pointed to the monitor: “Quick, quick—look at what’s happening now!” Maslach took a look and what she saw made her sick. A guard was screaming at a row of prisoners whose feet were shackled together and whose heads were covered with paper bags. They were on their way to the lavatory before going to bed. During the night, prisoners were forced to relieve themselves in buckets in their cells, which the guards allowed them to empty only when they felt like it. “Do you see that? Come on, look—it’s amazing stuff!” But Maslach had had enough. When Zimbardo, on leaving the prison, asked her what she thought of the experiment, she shouted at him: “What you are doing to those boys is a terrible thing!” Their discussion turned heated, and Zimbardo realized that all the people taking part in the experiment had internalized the destructive values of prison life. He decided then and there to end the experiment the next day.

The most important result of the Stanford Prison Experiment was to realize how powerful circumstances can be. As in the Milgram Experiment, totally ordinary students placed in an unfamiliar situation demonstrated completely unexpected behavior. Apparently, personality is no guide to behavior when people are in situations for which they do not know the rules. “Thus any deed that any human being has ever done, however horrible, is possible for any of us to do—under the right or wrong situational pressures,” wrote Zimbardo after the experiment. “That knowledge does not excuse evil; rather, it democratizes it, shares its blame among ordinary participants, rather than demonizes it.”

This uncomfortable revelation about the nature of people is hard to accept. When, in April 2003, American solders in Baghdad tortured Iraqi prisoners and released pictures of their actions to the public, the administration in Washington announced that the men and women involved represented only a few “bad apples.”

Because Zimbardo had his jail monitored around the clock by a videocamera, his study served as a forerunner to the reality-TV format—with the important difference that the study was not motivated by a quest for high ratings. But that came later. In January 2002 the BBC introduced The Experiment, a reality show intended to repeat the Stanford Prison Experiment before the eyes of millions of TV watchers. Zimbardo has stated that the results of the show, which was developed by two psychologists, might be questionable because the participants know they are being filmed the entire time.

In response to the the E-mails Zimbardo got from Germany after the film The Experiment was shown, he had his information about the experiment on his Web site translated into German, and also into Spanish and Italian. Zimbardo’s role might not have been heroic, but unlike the film, his experiment involved no rape or murder. In a follow-up study a year after the experiment, none of the participants showed negative side effects. Prisoner 8612, who was the first to break down, later became a psychologist in a district jail in San Francisco.




Philip Zimbardo’s web site on the Stanford Prison Experiment offers a dramatic diashow with videoclips on the progress of the experiment. Original material from the experiment (for example, the 17 prison rules and the description of the experiment given to the participants at the outset, as well as further studies and reactions, can be found at www.prisonexp.org/links.htm.

“Stanford-Prison-Experiment” is also a rock band in Los Angeles.

The German film “Das Experiment” (2001) was inspired by an experiment that took place at Stanford University.

Haney, C., et al. (1973), Interpersonal Dynamics in a Simulated Prison. International Journal of Criminology & Penology 1 (1), S. 69–97. Fulltext (PDF 1.4 MB)


1971 Galileo on the moon
As early as the 17th century Galileo Galilei showed through an elegant thought experiment that the velocity of a falling object is independent of its weight. But we have always had a hard time believing it. In everyday life, our experience is precisely the opposite: a bottle falls faster than a leaf, a hailstone faster than snowflake, a hammer faster than a feather. Of course, physics teachers tell us that the different velocities have something to do with air resistance, not with mass. But the power of what we observe with our own eyes is compelling.

That’s why on August 2, 1971, astronaut David Scott performed an experiment in front of live cameras. In the airless atmosphere of the Moon, Scott simultaneously let drop a feather and a hammer 40 times the feather’s weight. Both landed on the Moon’s surface at the same time. Although a NASA report on the Apollo 15 mission stated later that the result was known beforehand, it was still reassuring—especially considering that the success of the homeward journey depended critically on the theory being tested.




What falls faster on the moon: a feather or a hammer? See for yourself! (Film 00:47).

Allen, J. (1972), Apollo 15 Preliminary Science Report/Chapter 2: Summary of Scientific Results (SP-289), NASA, S. 11. Fulltext (PDF 53 MB!!)


2003 Robot encounters
Early behavioral researchers tried to fool animals with cheap imitations. Now, the robot age has come to the field of animal behavior. Researchers at Eoetvoes Loránd University in Budapest and the Sony Computer Science Laboratory in Paris wanted to find out whether dogs would accept Sony’s commercial robot AIBO as a companion. The researchers brought together 30 biological dogs and Sony’s 30-centimeter-long, one-and-a-half-kilogram technodog. For some tests, they covered AIBO with a piece of fur that they had let sit in a puppy’s sleeping box one day previously.

For comparison, the researchers also tested the dogs with a real puppy and a model car. After considering how much time the dogs spent looking at the robot and how they approached it, as well as the amount of barking, growling, and sniffing (fore and aft), the researchers concluded, “It seems that at present there are some serious limitations in using AIBO robots for behavioral tests with dogs.” The dogs did react to the robot, though much less strongly than to the puppy.

On their Web site, the researchers state that no animal was harmed during the experiments. AIBO suffered several attacks, but it still functioned perfectly. Nevertheless, the researchers caution against similar attempts at home. The warranty of the manufacturer does not cover injuries to AIBOs by real dogs.














How roughly the encounter between dog and robot can go for the robot is shown in a film (00:16) made by researchers from Eötvös Loránd Universität (Hungary) and Sony Computer Science Laboratory (Paris).

Kubinyi, E., et al. (2004), Social Behaviour of Dogs Encoutering AIBO, an Animal-Like Robot in a Neutral and in a Feeding Situation. Behavioural Processes 65, S. 231–239.


Falling cats
Dr. Pavlov only rings once
The 21-gram soul
Stairway to a banana
The living dog’s head
The curve of ecstasy
Mr. Skinner’s box
Vacation in the draft
Obedient to the end
Six degrees of separation
The professor’s prison
Galileo on the moon
Robot encounters