Nowadays, the American Psychological Association has a Code of Conduct in place when it comes to ethics in psychological experiments. Experimenters must adhere to various rules pertaining to everything from confidentiality to consent to overall beneficence. Review boards are in place to enforce these ethics. But the standards were not always so strict, which is how some of the most famous studies in psychology came about.

1. The Little Albert Experiment

At Johns Hopkins University in 1920, John B. Watson conducted a study of classical conditioning, a phenomenon that pairs a conditioned stimulus with an unconditioned stimulus until they produce the same result. This type of conditioning can create a response in a person or animal towards an object or sound that was previously neutral. Classical conditioning is commonly associated with Ivan Pavlov, who rang a bell every time he fed his dog until the mere sound of the bell caused his dog to salivate.

Watson tested classical conditioning on a 9-month-old baby he called Albert B. The young boy started the experiment loving animals, particularly a white rat. Watson started pairing the presence of the rat with the loud sound of a hammer hitting metal. Albert began to develop a fear of the white rat as well as most animals and furry objects. The experiment is considered particularly unethical today because Albert was never desensitized to the phobias that Watson produced in him. (The child died of an unrelated illness at age 6, so doctors were unable to determine if his phobias would have lasted into adulthood.)

2. Asch Conformity Experiments

Solomon Asch tested conformity at Swarthmore College in 1951 by putting a participant in a group of people whose task was to match line lengths. Each individual was expected to announce which of three lines was the closest in length to a reference line. But the participant was placed in a group of actors, who were all told to give the correct answer twice then switch to each saying the same incorrect answer. Asch wanted to see whether the participant would conform and start to give the wrong answer as well, knowing that he would otherwise be a single outlier.

Thirty-seven of the 50 participants agreed with the incorrect group despite physical evidence to the contrary. Asch used deception in his experiment without getting informed consent from his participants, so his study could not be replicated today.

3. The Bystander Effect

Some psychological experiments that were designed to test the bystander effect are considered unethical by today’s standards. In 1968, John Darley and Bibb Latané developed an interest in crime witnesses who did not take action. They were particularly intrigued by the murder of Kitty Genovese, a young woman whose murder was witnessed by many, but still not prevented.

The pair conducted a study at Columbia University in which they would give a participant a survey and leave him alone in a room to fill out the paper. Harmless smoke would start to seep into the room after a short amount of time. The study showed that the solo participant was much faster to report the smoke than participants who had the exact same experience, but were in a group.

The studies became progressively unethical by putting participants at risk of psychological harm. Darley and Latané played a recording of an actor pretending to have a seizure in the headphones of a person, who believed he or she was listening to an actual medical emergency that was taking place down the hall. Again, participants were much quicker to react when they thought they were the sole person who could hear the seizure.

4. The Milgram Experiment

Yale psychologist Stanley Milgram hoped to further understand how so many people came to participate in the cruel acts of the Holocaust. He theorized that people are generally inclined to obey authority figures, posing the question, “Could it be that Eichmann and his million accomplices in the Holocaust were just following orders? Could we call them all accomplices?” In 1961, he began to conduct experiments of obedience.

Participants were under the impression that they were part of a study of memory. Each trial had a pair divided into “teacher” and “learner,” but one person was an actor, so only one was a true participant. The drawing was rigged so that the participant always took the role of “teacher.” The two were moved into separate rooms and the “teacher” was given instructions. He or she pressed a button to shock the “learner” each time an incorrect answer was provided. These shocks would increase in voltage each time. Eventually, the actor would start to complain followed by more and more desperate screaming. Milgram learned that the majority of participants followed orders to continue delivering shocks despite the clear discomfort of the “learner.”

Had the shocks existed and been at the voltage they were labeled, the majority would have actually killed the “learner” in the next room. Having this fact revealed to the participant after the study concluded would be a clear example of psychological harm.

5. Harlow’s Monkey Experiments

In the 1950s, Harry Harlow of the University of Wisconsin tested infant dependency using rhesus monkeys in his experiments rather than human babies. The monkey was removed from its actual mother which was replaced with two “mothers,” one made of cloth and one made of wire. The cloth “mother” served no purpose other than its comforting feel whereas the wire “mother” fed the monkey through a bottle. The monkey spent the majority of his day next to the cloth “mother” and only around one hour a day next to the wire “mother,” despite the association between the wire model and food.

Harlow also used intimidation to prove that the monkey found the cloth “mother” to be superior. He would scare the infants and watch as the monkey ran towards the cloth model. Harlow also conducted experiments which isolated monkeys from other monkeys in order to show that those who did not learn to be part of the group at a young age were unable to assimilate and mate when they got older. Harlow’s experiments ceased in 1985 due to APA rules against the mistreatment of animals as well as humans. However, Department of Psychiatry Chair Ned H. Kalin, M.D. of the University of Wisconsin School of Medicine and Public Health has recently begun similar experiments that involve isolating infant monkeys and exposing them to frightening stimuli. He hopes to discover data on human anxiety, but is meeting with resistance from animal welfare organizations and the general public.

6. Learned Helplessness

The ethics of Martin Seligman’s experiments on learned helplessness would also be called into question today due to his mistreatment of animals. In 1965, Seligman and his team used dogs as subjects to test how one might perceive control. The group would place a dog on one side of a box that was divided in half by a low barrier. Then they would administer a shock, which was avoidable if the dog jumped over the barrier to the other half. Dogs quickly learned how to prevent themselves from being shocked.

Seligman’s group then harnessed a group of dogs and randomly administered shocks, which were completely unavoidable. The next day, these dogs were placed in the box with the barrier. Despite new circumstances that would have allowed them to escape the painful shocks, these dogs did not even try to jump over the barrier; they only cried and did not jump at all, demonstrating learned helplessness.

7. Robbers Cave Experiment

Muzafer Sherif conducted the Robbers Cave Experiment in the summer of 1954, testing group dynamics in the face of conflict. A group of preteen boys were brought to a summer camp, but they did not know that the counselors were actually psychological researchers. The boys were split into two groups, which were kept very separate. The groups only came into contact with each other when they were competing in sporting events or other activities.

The experimenters orchestrated increased tension between the two groups, particularly by keeping competitions close in points. Then, Sherif created problems, such as a water shortage, that would require both teams to unite and work together in order to achieve a goal. After a few of these, the groups became completely undivided and amicable.

Though the experiment seems simple and perhaps harmless, it would still be considered unethical today because Sherif used deception as the boys did not know they were participating in a psychological experiment. Sherif also did not have informed consent from participants.

8. The Monster Study

At the University of Iowa in 1939, Wendell Johnson and his team hoped to discover the cause of stuttering by attempting to turn orphans into stutterers. There were 22 young subjects, 12 of whom were non-stutterers. Half of the group experienced positive teaching whereas the other group dealt with negative reinforcement. The teachers continually told the latter group that they had stutters. No one in either group became stutterers at the end of the experiment, but those who received negative treatment did develop many of the self-esteem problems that stutterers often show. Perhaps Johnson’s interest in this phenomenon had to do with his own stutter as a child, but this study would never pass with a contemporary review board.

Johnson’s reputation as an unethical psychologist has not caused the University of Iowa to remove his name from its Speech and Hearing Clinic.

9. Blue Eyed versus Brown Eyed Students

Jane Elliott was not a psychologist, but she developed one of the most famously controversial exercises in 1968 by dividing students into a blue-eyed group and a brown-eyed group. Elliott was an elementary school teacher in Iowa, who was trying to give her students hands-on experience with discrimination the day after Martin Luther King Jr. was shot, but this exercise still has significance to psychology today. The famous exercise even transformed Elliott’s career into one centered around diversity training.

After dividing the class into groups, Elliott would cite phony scientific research claiming that one group was superior to the other. Throughout the day, the group would be treated as such. Elliott learned that it only took a day for the “superior” group to turn crueler and the “inferior” group to become more insecure. The blue eyed and brown eyed groups then switched so that all students endured the same prejudices.

Elliott’s exercise (which she repeated in 1969 and 1970) received plenty of public backlash, which is probably why it would not be replicated in a psychological experiment or classroom today. The main ethical concerns would be with deception and consent, though some of the original participants still regard the experiment as life-changing.

10. The Stanford Prison Experiment

In 1971, Philip Zimbardo of Stanford University conducted his famous prison experiment, which aimed to examine group behavior and the importance of roles. Zimbardo and his team picked a group of 24 male college students who were considered “healthy,” both physically and psychologically. The men had signed up to participate in a “psychological study of prison life,” which would pay them $15 per day. Half were randomly assigned to be prisoners and the other half were assigned to be prison guards. The experiment played out in the basement of the Stanford psychology department where Zimbardo’s team had created a makeshift prison. The experimenters went to great lengths to create a realistic experience for the prisoners, including fake arrests at the participants’ homes.

The prisoners were given a fairly standard introduction to prison life, which included being deloused and assigned an embarrassing uniform. The guards were given vague instructions that they should never be violent with the prisoners, but needed to stay in control. The first day passed without incident, but the prisoners rebelled on the second day by barricading themselves in their cells and ignoring the guards. This behavior shocked the guards and presumably led to the psychological abuse that followed. The guards started separating “good” and “bad” prisoners, and doled out punishments including push ups, solitary confinement, and public humiliation to rebellious prisoners.

Zimbardo explained, “In only a few days, our guards became sadistic and our prisoners became depressed and showed signs of extreme stress.” Two prisoners dropped out of the experiment; one eventually became a psychologist and a consultant for prisons. The experiment was originally supposed to last for two weeks, but it ended early when Zimbardo’s future wife, psychologist Christina Maslach, visited the experiment on the fifth day and told him, “I think it’s terrible what you’re doing to those boys.”

Despite the unethical experiment, Zimbardo is still a working psychologist today. He was even honored by the American Psychological Association with a Gold Medal Award for Life Achievement in the Science of Psychology in 2012.