The Most Famous Experiments in Psychology

The field of psychology is a very broad field composed of many smaller specialty areas. Each of these specialty areas has been bolstered over the years by research studies designed to prove or disprove theories and hypotheses that pique the interest of psychologists around the world and help us understand human behavior.

Although thousands upon thousands of studies are conducted each year in the many specialty areas of psychology, there are a handful that, over the years, have had a lasting impact on the psychological community as a whole. Some of these were conducted dutifully, staying within the bounds of ethical and practical guidelines

Others pushed the limits of human behavior during their psychological experiments and created controversies that still linger today. Still others were not designed as true psychological experiments, but ended up being beacons for the psychological community in proving or disproving theories.

In this article we show you the experiments that have marked the history of psychology

20 Most Important and Famous Experiments in Psychology

1-Pavlov’s dog

Pavlov’s experiment with dogs turned out to be one of the most important in all of psychology. His discoveries about conditioning gave rise to a whole new branch of psychological study.

Pavlov he started from the simple idea that there are things that a dog does not need to learn. Specifically, he observed that dogs do not learn to salivate when they see food. This reflex is “hard-wired” into the dog. In what became “behaviorist terms,” it is an unconditioned response (a stimulus-response connection that does not require learning).

Pavlov demonstrated that unconditioned responses exist in the animal by presenting a dog with a plate of food and then measuring its salivary secretions

In the experiment, Pavlov used a bell as a neutral stimulus (meaning that it does not elicit any innate response). Each time he gave food to his dogs, he also rang a bell. After several repetitions of this procedure, he tested the bell alone. What he discovered was that the bell alone caused an increase in salivation

The dog had learned to associate the bell and food and this learning created a new behavior, the dog salivated when it heard the bell. Because this response was learned (or conditioned), it is called a conditioned response. The neutral stimulus has become a conditioned stimulus.

This theory is known as classical conditioning (developed by the experimenter and psychologist John Watson) and involves learning to associate an unconditioned stimulus that already elicits a particular response (i.e., a reflex) with a new (conditioned) stimulus, so that the new stimulus elicits the same response.

2-Little Albert

The Little Albert experiment is considered to be one of the most unethical psychological experiments of all time. The experiment was conducted in 1920 by John Watson and Rosalie Rayner at Johns Hopkins University. The hypothesis was that, through a series of pairings, they could condition a nine-month-old child to develop irrational fear.

The experiment began by placing a white rat in front of the infant, who initially had no fear of the animal. Watson then produced a loud sound by striking a steel bar with a hammer each time little Albert was presented with the rat. After several pairings (the noise and the presentation of the white rat), the child began to cry and show signs of fear each time the rat appeared in the room. Watson also created similar conditioned reflexes with other common animals and objects (rabbits, Santa’s beard, etc.) until Albert feared them all.

This study demonstrated that classical conditioning works in humans. One of the most important implications of this finding is that adult fears are often related to early childhood experiences.

3-Cognitive Dissonance

The concept of cognitive dissonance refers to a situation involving conflicting attitudes, beliefs, or behaviors. This conflict produces an inherent sense of discomfort that leads to a change in one of the attitudes, beliefs, or behaviors to minimize or eliminate the discomfort and restore balance.

Cognitive dissonance was first investigated by Leon Festinger the study of cognitive dissonance was first investigated by the University of California, Berkeley, after an observational study of a sect that believed that the earth was going to be destroyed by a flood. Out of this study came an intriguing experiment by Festinger and Carlsmith in which participants were asked to perform a series of boring tasks (such as turning pegs on a pegboard for an hour)

The participants’ initial attitude toward this task was very negative. Next, they were paid $1 or $20 to tell a participant waiting in the lobby that the tasks were really interesting.

Almost all participants agreed to enter the waiting room and persuade the next participant that the boring experiment would be fun. When participants were later asked to evaluate the experiment, participants who were paid only $1 rated the tedious task as more fun and enjoyable than participants who were paid $20 to lie

Being paid only $1 is not enough incentive to lie, so those who were paid $1 experienced dissonance. They were only able to overcome that cognitive dissonance by coming to believe that the tasks were actually interesting and enjoyable. Being paid $20 provided a reason to turn the pegs and thus no dissonance.

4-Learned Disapprehension

In 1965, Martin Seligman and his colleagues were investigating classical conditioning, the process by which an animal or human associates one thing with another.

Seligman’s experiment consisted of ringing a bell and then administering a mild shock to a dog. After a series of pairings, the dog reacted to the shock even before it occurred: as soon as the dog heard the bell, it reacted as if it had already received the shock

In the course of this study, something unexpected happened. Each dog was placed in a large box that was divided in half with a low fence and the dog could easily see and jump over the fence. The ground on one side of the fence was electrified, but not the other side. Seligman placed each dog on the electrified side and administered a mild shock. He expected the dog to jump to the non-electrified side of the fence. In an unexpected twist, the dogs simply lay down

The hypothesis was that, because the dogs learned in the first part of the experiment that they could do nothing to avoid the shocks, they gave up in the second part of the experiment. To test this hypothesis, the experimenters brought in a new group of animals and found that the dogs with no history in the experiment jumped the fence.

This condition was described as learned helplessness, in which a human or animal does not try to get out of a negative situation because the past has taught them that they are helpless.

5-The Stanford Prison

One of the most widely cited experiments in the field of psychology is the Stanford Prison Experiment, in which the psychology professor Philip Zimbardo set out to study role-taking in an artificial situation.

The Stanford Prison Experiment was designed to study the behavior of “normal” individuals when assigned a prisoner or guard role. Undergraduates were recruited to participate and were assigned the roles of “guard” or “inmate” and Zimbardo played the role of warden

The basement of the psychology building was the prison setting and great care was taken to make it look and feel as realistic as possible. The guards were asked to run a prison for two weeks. They were asked not to physically harm any of the inmates during the study

After a few days, the prison guards became very verbally abusive to the inmates and many of the inmates became submissive to those in authority roles. The Stanford Prison Experiment inevitably had to be cancelled because some of the participants showed disturbing signs of mental breakdown.

Although the experiment was conducted in a highly unethical manner, many psychologists believe that the results demonstrated that human behavior depends on the situation and that people adapt to certain roles if the conditions are right. The Stanford Prison Experiment remains one of the most famous psychological experiments of all time.

6-Stanley Milgram Experiment

The 1961 study conducted by the Yale University psychologist Stanley Milgram was designed to measure people’s willingness to obey authority figures when ordered to perform acts that conflicted with their morals. The study was based on the premise that human beings intrinsically accept instructions from authority figures from a very early age.

Participants were told that they were going to participate in a memory study. They were asked to watch another person (who was actually an actor) take a memory test and were instructed to press a button that gave an electric shock each time the person got the answer wrong (the actor did not actually receive the shocks, but pretended to do so). Participants were told to play the role of the “teacher” and administer electric shocks to the “student,” who was supposedly in a different room, each time he or she answered a question incorrectly

The experimenters asked the participants to keep increasing the shocks, and most of them complied even though the individual completing the memory test appeared to suffer greatly. Despite these protests, many participants continued the experiment when urged to do so by the authority figure, increasing the voltage after each incorrect answer until some ended up administering what would be lethal electric shocks.

This experiment demonstrated that humans are conditioned to obey authority and tend to do so even if it goes against their natural morals or common sense.

7-Hawthorne Effect

The Hawthorne effect comes from a 1955 study by Henry Landsberger. This effect is a simple premise that human subjects in an experiment change their behavior simply because they are being studied.

Landsberger conducted the study by analyzing data from experiments conducted between 1924 and 1932, by Elton Mayo, at the Hawthorne Works near Chicago. The company had commissioned studies to assess whether the level of light inside a building changed worker productivity

What Mayo found was that the level of light made no difference to productivity, as workers increased their performance each time the amount of light was changed from a low level to a high level, or vice versa. The researchers observed a tendency for the workers’ efficiency level to increase when either variable was manipulated

The study showed that performance changed simply because the workers were aware that they were under observation. The conclusion was that workers felt important because they took pleasure in being singled out, and productivity increased as a result. Being singled out was the factor that determined the increase in productivity, not the change in lighting levels or any of the other factors they experimented with

The Hawthorne effect has become one of the most difficult built-in biases to eliminate or account for in the design of any experiment in psychology and elsewhere.

8-The Magic Number Seven

Frequently referred to as“Miller’s Law,” the Magic Number Seven experiment states that the number of objects an average human can retain in working memory is 7 ± 2. What this means is that human memory capacity usually includes strings of words or concepts ranging from 5 to 9. This information about the limits of information processing capacity became one of the most cited works in psychology.

The magic number seven experiment was published in 1956 by cognitive psychologist George A. Miller of Princeton University’s Department of Psychology in Psychological Review

In the article, Miller discussed a coincidence between the limits of unidimensional absolute judgment and the limits of short-term memory. In a unidimensional absolute judgment task, a person is presented with a series of stimuli that vary along one dimension (such as 10 different tones that vary only in pitch) and responds to each stimulus with a corresponding (previously learned) response

Performance is near perfect up to five or six different stimuli, but decreases as the number of different stimuli increases. This means that the maximum performance of a human being in one-dimensional absolute judgment can be described as an information store with a maximum capacity of about 2 to 3 bits of information, with the ability to distinguish between four and eight alternatives.

9-The Cave of Thieves

This experiment, which studied group conflict, is considered by most to be outside the lines of what is considered ethically correct.

In 1954, researchers at the University of Oklahoma assigned 22 eleven- and twelve-year-old boys from similar backgrounds into two groups. The two groups were taken to separate areas of a summer camp where they could bond as social units. The groups were housed in separate cabins and neither group knew of the existence of the other for an entire week

During that time, the boys bonded with their cabin mates. Once contact was allowed between the two groups, they showed clear signs of prejudice and hostility toward each other, even though they had been given only a very short time to develop their social group. To increase the conflict between the groups, the experimenters had them compete against each other in a series of activities

This created even more hostility and eventually the groups refused to eat in the same room. The final phase of the experiment involved turning the rival groups into friends

The fun activities the experimenters had planned, such as shooting firecrackers and watching movies, did not work initially, so they created teamwork exercises in which the two groups were forced to collaborate.

At the end of the experiment, the boys decided to ride home on the same bus, proving that conflicts can be resolved and prejudices overcome through cooperation.

Many critics have compared this study to Golding’s novel Lord of the Flies as a classic example of prejudice and conflict resolution.

10-The Bobo Doll

In the early 1960s, a great debate began over how genetics, environmental factors, and social learning determine child development. This debate still endures and is commonly referred to as the Nature vs. Nurture Debate

Albert Bandura conducted the Bobo Doll Experiment to demonstrate that human behavior is largely based on social imitation rather than inherited genetic factors.

In his groundbreaking study he separated participants into three groups: one was exposed to a video of an adult displaying aggressive behavior toward a Bobo doll; another was exposed to a video of a passive adult playing with the Bobo doll; and the third formed a control group. The children watched their assigned video and were then sent to a room with the same doll they had seen in the video (with the exception of those in the control group)

The researcher found that the children exposed to the aggressive model were more likely to show aggressive behavior toward the doll, while the other groups showed little imitative aggressive behavior. For children exposed to the aggressive model, the number of derived physical aggression shown by boys was 38.2 and 12.7 for girls.

The study also showed that boys showed more aggression when exposed to aggressive male models than boys exposed to aggressive female models. When exposed to aggressive male role models, the number of aggressive instances exhibited by boys averaged 104, compared to 48.4 aggressive instances exhibited by boys who were exposed to aggressive female role models. Although the results for girls are similar, the results are less dramatic

When exposed to aggressive female models, the number of aggressive instances exhibited by girls is 57.7 on average, compared to 36.3 aggressive instances exhibited by girls who were exposed to aggressive male models. The results regarding gender differences strongly supported Bandura’s secondary prediction that boys will be more influenced by same-sex models.

The Bobo Doll Experiment showed an innovative way to study human behavior and its influences.

11-Asch Experiment

Dr. Solomon Asch conducted a pioneering study designed to assess the likelihood that a person would conform to a norm when pressured to do so.

A group of participants were shown pictures with lines of different lengths and then asked a simple question: which line is longer? The tricky thing about this study is that in each group only one person was a real participant. The others were actors with a script. Most of the actors were instructed to give the wrong answer. Interestingly, the one true participant almost always agreed with the majority, even though he knew he was giving the wrong answer.

The results of this study are important when studying social interactions between individuals in groups. This study is a famous example of the temptation many of us experience to conform to a norm during group situations and demonstrated that people are often more concerned with being the same as others than with being right

It is still recognized as one of the most influential psychological experiments in understanding human behavior.

12-Schater and Singer’s Emotion Experiment

In 1962, Schachter and Singer conducted a pioneering experiment to demonstrate their theory of emotion.

In the study, a group of 184 male participants were injected with epinephrine, a hormone that induces arousal, including increased heartbeat, tremors, and rapid breathing. The research participants were told that they were going to be injected with a new drug to test their eyesight

The first group of participants was informed of the possible side effects the injection might cause, while the second group of participants was not. The participants were then placed in a room with someone they thought was another participant, but who was actually a confederate in the experiment. The confederate acted in two ways: elated or angry. Participants who had not been informed of the effects of the injection were more likely to feel happier or angrier than those who had been informed.

What Schachter and Singer were trying to understand was how cognition or thoughts influence human emotions. Their study illustrates the importance of how people interpret their physiological states, which form an important component of their emotions

Although his cognitive theory of emotional arousal dominated the field for two decades, it has been criticized for two main reasons: the effect size observed in the experiment was not that significant and other researchers had difficulty repeating the experiment.

13-The Good Samaritan

In 1973, John Darley and Daniel Batson created an experiment to investigate the possible causes underlying altruistic behavior. The experiment’s researchers posed three hypotheses that they wanted to test:

• People who think about religion and higher principles would be no more inclined to exhibit helping behavior than laymen.
• People in a hurry would be much less likely to show helping behavior.
• People who are religious for personal gain would be less likely to show helping behavior than people who are religious because they want to gain some spiritual and personal insight into the meaning of life.

The student participants were given some religious teachings and instructions and then told to travel from one building to another. Between the two buildings was an injured man who appeared to be in dire need of help. The first variable tested was the degree of urgency impressed on the subjects: some were told not to rush and others were informed that speed was of the essence.

The results of the experiment were intriguing, as the subject’s haste turned out to be the primary factor. When the subject was not in a hurry, nearly two-thirds of the people stopped to render aid. When the subject was in a hurry, the proportion dropped to one in ten. People who were going to deliver a speech about helping others were almost twice as likely to help as those who delivered other sermons, showing that the individual’s thoughts were a factor in determining helping behavior

Religious beliefs do not appear to have much influence on the results; being religious out of personal interest or as part of a spiritual quest does not appear to have a noticeable impact on the amount of helping behavior displayed.

14-False Consensus Effect

In 1977, a Stanford University social psychology professor named Lee Ross conducted an experiment that, in simple terms, focuses on how people may incorrectly conclude that others think the same as they do, or form a“false consensus” about the beliefs and preferences of others

Ross conducted the study to outline how the“false consensus effect” works in humans.

In the first part of the study, participants were asked to read about situations in which a conflict occurred, and then were told two alternative ways to respond to the situation. They were asked to do three things:

• Guess which option other people would choose
• Tell which option they would choose themselves
• Describe the attributes of the person likely to choose each of the two options

The study showed that most subjects believed that other people would do the same thing they did, regardless of which of the two answers they chose themselves. This phenomenon is known as the false consensus effect, in which an individual thinks that others think the same as them when this may not be the case

The second observation to emerge from this important study is that when participants were asked to describe the attributes of people who were likely to make the opposite choice to their own, they made bold and sometimes negative predictions about the personalities of those who did not share their choice.

15-Marshmallow Experiment

Walter Mischel of Stanford University set out to study whether delayed gratification can be an indicator of future success.

In his 1972 Marshmallow Experiment , children between the ages of four and six were brought into a room where a marshmallow was placed on the table in front of them. Before leaving each of the children alone in the room, the experimenter informed them that they would receive a second marshmallow if the first one was still on the table when they returned in 15 minutes. The examiner recorded how long each child resisted eating the marshmallow and noted whether it correlated with the child’s success as an adult

A small number of the 600 children ate the marshmallow immediately and one-third delayed gratification long enough to receive the second marshmallow.

In follow-up studies, Mischel found that those who delayed gratification were significantly more competent and scored higher on the SAT than their peers, meaning that this trait likely remains with the person for life

Although this study seems simplistic, the results highlight some of the fundamental differences in individual traits that can predict success.

16-Halo Effect

The Halo Effect states that people generally assume that people who are physically attractive are more likely to be intelligent, friendly, and show good judgment. To test their theory, Nisbett and DeCamp Wilson created a study to demonstrate that people are unaware of the nature of the Halo Effect, and that it influences their personal judgments, inferences, and production of more complex social behavior.

In the experiment, undergraduate students were the research participants and were asked to evaluate a psychology instructor while watching him in a videotaped interview. Students were randomly assigned to one of two groups, and each group was shown one of two different interviews with the same instructor, who is a native French-speaking Belgian who spoke English with a rather noticeable accent

In the first video, the teacher presented himself as friendly, respectful of his students’ intelligence and motives, flexible in his approach to teaching, and enthusiastic about his subject. In the second interview, he came across as much more unfriendly. He came across as cold and distrustful of students and was quite rigid in his teaching style.

After watching the videos, subjects were asked to rate the teacher on his physical appearance, mannerisms, and accent, although his mannerisms and accent were the same in both versions of the videos. Subjects were asked to rate the professor on an 8-point scale ranging from “I like him a lot” to “I don’t like him at all.” Subjects were also told that the researchers were interested in knowing “to what extent their liking of the teacher influenced the ratings they had just made.” The other subjects were asked to identify to what extent the characteristics they had just rated influenced their liking of the teacher.

After answering the questionnaire, the respondents were puzzled about their reactions to the videotapes and the questionnaire items. The students had no idea why they had rated a teacher more highly. Most said that how much they liked the teacher for what he or she said had not affected their assessment of their individual characteristics at all

What is interesting about this study is that people may understand the phenomenon, but they are not aware of when it occurs. Without realizing it, humans make judgments and even when it is pointed out to them, they can still deny that it is a product of the halo effect phenomenon.

17-The Invisible Gorilla

In 1999, Simons and Chabris conducted their famous consciousness test at Harvard University.

Study participants were asked to watch a video and count how many passes occurred between basketball players on the white team. The video moves at a moderate pace and keeping track of the passes is a relatively easy task. What most people don’t notice in the midst of their counting is that in the middle of the test, a man in a gorilla suit entered the court and stood in the middle before exiting the screen.

The study found that most of the subjects did not notice the gorilla at all, showing that humans often overestimate their ability to multitask.

What the study was intended to show is that when people are asked to attend to one task, they focus so much on that one item that they may overlook other important details.

18-The Fiddler in the subway

The Washington Post staff conducted an interesting study to see how observant people are of what is going on around them.

During the study, pedestrians rushed by without noticing that the musician playing at the entrance to the subway stop was Grammy winner Joshua Bell, who, two days before his subway performance, sold out a Boston theater where seats cost an average of $100

He played one of the most intricate pieces ever written on a violin worth $3.5 million. In the 45 minutes that the musician played his violin, only 6 people stopped and stayed for a while. About 20 gave him money, but they kept walking at their normal pace. He collected $32.

The study and subsequent article organized by the Washington Post were part of a social experiment looking at people’s perception, taste and priorities

Gene Weingarten wrote about the Washington Post social experiment (“In a banal environment at an inopportune time, would beauty transcend?“) and subsequently won a Pulitzer Prize for his article. Some of the questions the article addresses are Do we perceive beauty? Do we stop to appreciate it? Do we recognize talent in an unexpected context? It turns out that many of us are not as perceptive to our surroundings as we would like to think.

19-Loftus and Palmer Experiment

Loftus and Palmer set out to demonstrate how deceptive memories can be. The 1974 Car Accident Experiment was designed to assess whether asking questions in a particular way could influence participants’ memories by distorting their memory of a particular event.

Participants viewed slides of a car accident and were asked to describe what happened as if they were eyewitnesses to the scene. Participants were divided into two groups and each group was asked a different formulation, such as, “How fast was the car going at the time of impact?” versus “How fast was the car going when it collided with the other car?” The experimenters found that the use of different verbs affected participants’ memories of the accident, demonstrating that memory can be easily distorted.

This research suggests that memory can be easily manipulated by the technique of interrogation, meaning that information gathered after the event can be merged with the original memory, causing an incorrect recollection or a reconstructive memory. The addition of false details to the memory of an event is now called confabulation.

This concept has very important implications for the questions used in police interrogations of eyewitnesses.

20- The Visual Cliff

In 1959, psychologists Eleanor Gibson and Richard Walk set out to study depth perception in infants. They wanted to know if depth perception is a learned behavior or if it is something we are born with. To study this, Gibson and Walk conducted the visual cliff experiment.

Gibson and Walk studied 36 infants between six and 14 months of age, all of whom could crawl. They placed the babies one at a time on a visual cliff, which is this device seen above

A visual cliff was created using a large glass table that was raised about 30 centimeters off the ground. Half of the glass table had a checkered pattern underneath to create the appearance of a “shallow side.” To create a “deep side,” a checkerboard pattern was created on the floor; this side is the visual cliff

Although the glass table extends across the entire surface, the placement of the checkerboard pattern on the floor creates the illusion of a sudden drop-off. The researchers placed a one-foot-wide center tabletop between the shallow side and the deep side. Gibson and Walk discovered the following:

• Nine of the babies did not move off the center board.
• All of the 27 babies who did move crossed over to the shallow side when their mothers called them from the shallow side.
• Three of the infants crawled off the visual cliff toward their mother when called from the deep side.
• When called from the deep side, the remaining 24 infants either crawled toward the shallow side or cried because they could not cross the visual cliff and reach their mother.

What this study helped to demonstrate is that depth perception is probably an innate train in humans.