Psychologists often use experiments to answer humanity’s most difficult questions. After the atrocities of Nazi Germany in World War II, many wondered how people could follow the orders to perform such horrific actions.

Yale researcher Stanley Milgram devised an experiment around the following question: “Could it be that [Adolf] Eichmann and his million accomplices in the Holocaust were just following orders? Could we call them all accomplices?”

[Adolf Eichmann was one of the major organizers of the Holocaust.]

He found that other populations would follow orders to harm people despite the orders conflicting with their personal morals. By studying this social phenomenon, scientists were able to surmise that atrocities committed during the war were not endemic to German soldiers, as initially believed.

Throughout history, other psychology experiments have tried to address specific issues to foster better understanding of human behavior.

Here is a look at five notable experiments from the second half of the 20th century to present day:

Bobo Doll Experiment

Conducted in 1961 by scientist Albert Bandura, this experiment sought to prove human behavior was learned through social imitation, rather than inherited genetically. Bandura hypothesized children would mimic an adult’s behavior if they trusted the adult. He chose to use a Bobo doll, a roughly 5-foot-tall inflatable toy weighted at the bottom that bounces back to standing upright after being struck.

One group of children did not witness any adult interaction with the toy. Another group watched an adult behave aggressively. Bandura’s experiment found that children exposed to aggression were more likely to imitate the behavior and boys were three times more likely to mimic violence than girls.

Bystander Effect

In the 1960s, John Darley and Bibb Latané sought to measure how much time elapsed before bystanders reacted and either intervened or ignored the need for help when an emergency situation involving a group or an individual was staged. The researchers were inspired by the murder of Kitty Genovese in 1964, which became infamous after The New York Times reported that there were 38 witnesses to her murder and none of them tried to help.

Although the Kitty Genovese phenomenon was debunked later by the Times itself, it caused Darley and Latané's discovery of The Bystander Effect. They demonstrated that a larger number of bystanders diminished the chances any of them would offer help. The Bystander Effect has continued to be replicated for years.

Halo Effect

The “halo effect” is a famous social psychology finding which suggests global or group evaluations about an individual can influence judgments about a specific trait. For example, a likable person is often perceived to be intelligent. The term, originally coined by psychology Edward Thorndike, is a type of confirmation bias.

In the 1970s, researchers Richard Nisbett and Timothy DeCamp Wilson performed an experiment to demonstrate this by showing two groups of students the same lecture, but changing the demeanor of the lecturer from one group to the next. When the lecturer appeared friendly, students responded more favorably than the group who saw the lecturer who appeared cold and distant.

The Chameleon Effect

Also referred to as “unintentional mirroring,” the Chameleon Effect is believed to be a natural tendency for one person to imitate another person based on how well they get along without any realization that it’s happening. Tanya Chartrand and John Bargh from New York University studied this phenomenon in the 1990s. They interviewed participants individually while affecting different mannerisms throughout the talk to gauge the bond that developed.

During two follow-up talks, the scientists mimicked the posture and other mannerisms of some test subjects. The participants mimicked the scientists more in the first experiment and found the scientists more likable when they mimicked their own mannerisms in the follow-ups. Those participants who weren’t mimicked had a more neutral opinion of the scientists.

The Volkswagen Fun Theory

In 2009, advertising agency DDB Stockholm created an initiative on behalf of car manufacturer Volkswagen. The company came up with the “Fun Theory,” conducting three experiments to see whether people might choose to change behavior and do something based on how much fun it was to do, such as recycling, throwing away trash or taking stairs versus an escalator.

In one instance, a set of stairs next to an escalator was decorated to look like piano keys with accompanying notes for each step a person took while traversing the stairs. The experiment found 66% more people chose the stairs than usual. In another, a trash bin with sound effects when people deposited litter collected more trash than nearby bins.

Though these were part of an advertising campaign rather than a scientific experiment, the results indicate people may be more inclined to perform a task such as taking stairs instead of an escalator if it appears to be fun.

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