experiment summary definition

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Experiment Definition in Science – What Is a Science Experiment?

In science, an experiment is simply a test of a hypothesis in the scientific method . It is a controlled examination of cause and effect. Here is a look at what a science experiment is (and is not), the key factors in an experiment, examples, and types of experiments.

Experiment Definition in Science

By definition, an experiment is a procedure that tests a hypothesis. A hypothesis, in turn, is a prediction of cause and effect or the predicted outcome of changing one factor of a situation. Both the hypothesis and experiment are components of the scientific method. The steps of the scientific method are:

• Make observations.
• Ask a question or identify a problem.
• State a hypothesis.
• Perform an experiment that tests the hypothesis.
• Based on the results of the experiment, either accept or reject the hypothesis.
• Draw conclusions and report the outcome of the experiment.

Key Parts of an Experiment

The two key parts of an experiment are the independent and dependent variables. The independent variable is the one factor that you control or change in an experiment. The dependent variable is the factor that you measure that responds to the independent variable. An experiment often includes other types of variables , but at its heart, it’s all about the relationship between the independent and dependent variable.

Examples of Experiments

Fertilizer and plant size.

For example, you think a certain fertilizer helps plants grow better. You’ve watched your plants grow and they seem to do better when they have the fertilizer compared to when they don’t. But, observations are only the beginning of science. So, you state a hypothesis: Adding fertilizer increases plant size. Note, you could have stated the hypothesis in different ways. Maybe you think the fertilizer increases plant mass or fruit production, for example. However you state the hypothesis, it includes both the independent and dependent variables. In this case, the independent variable is the presence or absence of fertilizer. The dependent variable is the response to the independent variable, which is the size of the plants.

Now that you have a hypothesis, the next step is designing an experiment that tests it. Experimental design is very important because the way you conduct an experiment influences its outcome. For example, if you use too small of an amount of fertilizer you may see no effect from the treatment. Or, if you dump an entire container of fertilizer on a plant you could kill it! So, recording the steps of the experiment help you judge the outcome of the experiment and aid others who come after you and examine your work. Other factors that might influence your results might include the species of plant and duration of the treatment. Record any conditions that might affect the outcome. Ideally, you want the only difference between your two groups of plants to be whether or not they receive fertilizer. Then, measure the height of the plants and see if there is a difference between the two groups.

Salt and Cookies

You don’t need a lab for an experiment. For example, consider a baking experiment. Let’s say you like the flavor of salt in your cookies, but you’re pretty sure the batch you made using extra salt fell a bit flat. If you double the amount of salt in a recipe, will it affect their size? Here, the independent variable is the amount of salt in the recipe and the dependent variable is cookie size.

Test this hypothesis with an experiment. Bake cookies using the normal recipe (your control group ) and bake some using twice the salt (the experimental group). Make sure it’s the exact same recipe. Bake the cookies at the same temperature and for the same time. Only change the amount of salt in the recipe. Then measure the height or diameter of the cookies and decide whether to accept or reject the hypothesis.

Examples of Things That Are Not Experiments

Based on the examples of experiments, you should see what is not an experiment:

• Making observations does not constitute an experiment. Initial observations often lead to an experiment, but are not a substitute for one.
• Making a model is not an experiment.
• Neither is making a poster.
• Just trying something to see what happens is not an experiment. You need a hypothesis or prediction about the outcome.
• Changing a lot of things at once isn’t an experiment. You only have one independent and one dependent variable. However, in an experiment, you might suspect the independent variable has an effect on a separate. So, you design a new experiment to test this.

Types of Experiments

There are three main types of experiments: controlled experiments, natural experiments, and field experiments,

• Controlled experiment : A controlled experiment compares two groups of samples that differ only in independent variable. For example, a drug trial compares the effect of a group taking a placebo (control group) against those getting the drug (the treatment group). Experiments in a lab or home generally are controlled experiments
• Natural experiment : Another name for a natural experiment is a quasi-experiment. In this type of experiment, the researcher does not directly control the independent variable, plus there may be other variables at play. Here, the goal is establishing a correlation between the independent and dependent variable. For example, in the formation of new elements a scientist hypothesizes that a certain collision between particles creates a new atom. But, other outcomes may be possible. Or, perhaps only decay products are observed that indicate the element, and not the new atom itself. Many fields of science rely on natural experiments, since controlled experiments aren’t always possible.
• Field experiment : While a controlled experiments takes place in a lab or other controlled setting, a field experiment occurs in a natural setting. Some phenomena cannot be readily studied in a lab or else the setting exerts an influence that affects the results. So, a field experiment may have higher validity. However, since the setting is not controlled, it is also subject to external factors and potential contamination. For example, if you study whether a certain plumage color affects bird mate selection, a field experiment in a natural environment eliminates the stressors of an artificial environment. Yet, other factors that could be controlled in a lab may influence results. For example, nutrition and health are controlled in a lab, but not in the field.
• Bailey, R.A. (2008). Design of Comparative Experiments . Cambridge: Cambridge University Press. ISBN 9780521683579.
• di Francia, G. Toraldo (1981). The Investigation of the Physical World . Cambridge University Press. ISBN 0-521-29925-X.
• Hinkelmann, Klaus; Kempthorne, Oscar (2008). Design and Analysis of Experiments. Volume I: Introduction to Experimental Design (2nd ed.). Wiley. ISBN 978-0-471-72756-9.
• Holland, Paul W. (December 1986). “Statistics and Causal Inference”.  Journal of the American Statistical Association . 81 (396): 945–960. doi: 10.2307/2289064
• Stohr-Hunt, Patricia (1996). “An Analysis of Frequency of Hands-on Experience and Science Achievement”. Journal of Research in Science Teaching . 33 (1): 101–109. doi: 10.1002/(SICI)1098-2736(199601)33:1<101::AID-TEA6>3.0.CO;2-Z

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Chapter 12: Statistics in Practice

Back to chapter, what is an experiment, next video 12.2: study design in statistics.

An experiment is a systematic approach to prove or disprove a hypothesis and uncover new knowledge. Statistics is often used to interpret the results of an experiment.

Consider an experiment to determine the relationship between mobile phone usage and sleep quality among college students.

These students, referred to as subjects, are divided into two groups. One is experimental, while the other is a control group. Only the experimental group is allowed to use mobile phones thirty minutes before bedtime, while the control group acts as the standard of comparison.

Researchers then record the number of hours of sleep and sleep latency or variables – the characteristics of subjects that are examined, measured, and interpreted.

Variables are of two types: dependent and independent.

Mobile phone usage is an independent variable controlled by researchers, and sleep hours and sleep latency are dependent variables that are measured and recorded.

Finally, hypothesis testing is used to infer whether the observed changes in sleep quality in the experimental group over the control group are statistically significant.

An experiment is a planned activity carried out under controlled conditions. The purpose of an experiment is to investigate the relationship between two variables. When one variable causes change in another, we call the first variable the explanatory or independent variable. The affected variable is called the response or dependent variable. In a randomized experiment, the researcher manipulates values of the explanatory variable and measures the resulting changes in the response variable. The different values of the explanatory variable are called treatments. An experimental unit is a single object or individual to be measured.

You want to investigate the effectiveness of vitamin E in preventing disease. You recruit a group of subjects and ask them if they regularly take vitamin E. You notice that the subjects who take vitamin E exhibit better health on average than those who do not. Does this prove that vitamin E is effective in disease prevention? It does not. There are many differences between the two groups compared in addition to vitamin E consumption. People who take vitamin E regularly often take other steps to improve their health: exercise, diet, other vitamin supplements, and choosing not to smoke. Any one of these factors could be influencing health. As described, this study does not prove that vitamin E is the key to disease prevention.

Additional variables that can cloud a study are called lurking variables. In order to prove that the explanatory variable is causing a change in the response variable, it is necessary to isolate the explanatory variable. The researcher must design her experiment in such a way that there is only one difference between the groups being compared: the planned treatments. This is accomplished by the random assignment of experimental units to treatment groups. When subjects are assigned treatments randomly, all of the potential lurking variables are spread equally among the groups. At this point, the only difference between groups is the one imposed by the researcher. Therefore, different outcomes measured in the response variable must be a direct result of the different treatments. In this way, an experiment can prove a cause-and-effect connection between the explanatory and response variables.

This text is adapted from Openstax, Introductory Statistics, Section 1.4 Experimental Design and Ethics

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Definition of experiment

 (Entry 1 of 2)

Definition of experiment  (Entry 2 of 2)

intransitive verb

• experimentation

Examples of experiment in a Sentence

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'experiment.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History

Middle English, "testing, proof, remedy," borrowed from Anglo-French esperiment, borrowed from Latin experīmentum "testing, experience, proof," from experīrī "to put to the test, attempt, have experience of, undergo" + -mentum -ment — more at experience entry 1

verbal derivative of experiment entry 1

14th century, in the meaning defined at sense 1a

1787, in the meaning defined above

Phrases Containing experiment

• control experiment
• controlled experiment
• experiment station
• pre - experiment
• thought experiment

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“Experiment.” Merriam-Webster.com Dictionary , Merriam-Webster, https://www.merriam-webster.com/dictionary/experiment. Accessed 29 Aug. 2024.

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Milgram experiment

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• Open University - OpenLearn - Psychological research, obedience and ethics: 1 Milgram’s obedience study
• Social Science LibreTexts - The Milgram Experiment- The Power of Authority
• Verywell Mind - What was the Milgram Experiment?
• BCcampus Open Publishing - Ethics in Law Enforcement - The Milgram Experiment
• Nature - Modern Milgram experiment sheds light on power of authority
• SimplyPsychology - Stanley Milgram Shock Experiment: Summary, Results, & Ethics
• University of California - College of Natural Resources - Milgrams Experiment on Obedience to Authority

Milgram experiment , controversial series of experiments examining obedience to authority conducted by social psychologist Stanley Milgram . In the experiment, an authority figure, the conductor of the experiment, would instruct a volunteer participant, labeled the “teacher,” to administer painful, even dangerous, electric shocks to the “learner,” who was actually an actor. Although the shocks were faked, the experiments are widely considered unethical today due to the lack of proper disclosure, informed consent, and subsequent debriefing related to the deception and trauma experienced by the teachers. Some of Milgram’s conclusions have been called into question. Nevertheless, the experiments and their results have been widely cited for their insight into how average people respond to authority.

Milgram conducted his experiments as an assistant professor at Yale University in the early 1960s. In 1961 he began to recruit men from New Haven , Connecticut , for participation in a study he claimed would be focused on memory and learning . The recruits were paid $4.50 at the beginning of the study and were generally between the ages of 20 and 50 and from a variety of employment backgrounds. When they volunteered, they were told that the experiment would test the effect of punishment on learning ability. In truth, the volunteers were the subjects of an experiment on obedience to authority. In all, about 780 people, only about 40 of them women, participated in the experiments, and Milgram published his results in 1963.

Volunteers were told that they would be randomly assigned either a “teacher” or “learner” role, with each teacher administering electric shocks to a learner in another room if the learner failed to answer questions correctly. In actuality, the random draw was fixed so that all the volunteer participants were assigned to the teacher role and the actors were assigned to the learner role. The teachers were then instructed in the electroshock “punishment” they would be administering, with 30 shock levels ranging from 15 to 450 volts. The different shock levels were labeled with descriptions of their effects, such as “Slight Shock,” “Intense Shock,” and “Danger: Severe Shock,” with the final label a grim “XXX.” Each teacher was given a 45-volt shock themselves so that they would better understand the punishment they believed the learner would be receiving. Teachers were then given a series of questions for the learner to answer, with each incorrect answer generally earning the learner a progressively stronger shock. The actor portraying the learner, who was seated out of sight of the teacher, had pre-recorded responses to these shocks that ranged from grunts of pain to screaming and pleading, claims of suffering a heart condition, and eventually dead silence. The experimenter, acting as an authority figure, would encourage the teachers to continue administering shocks, telling them with scripted responses that the experiment must continue despite the reactions of the learner. The infamous result of these experiments was that a disturbingly high number of the teachers were willing to proceed to the maximum voltage level, despite the pleas of the learner and the supposed danger of proceeding.

Milgram’s interest in the subject of authority, and his dark view of the results of his experiments, were deeply informed by his Jewish identity and the context of the Holocaust , which had occurred only a few years before. He had expected that Americans, known for their individualism , would differ from Germans in their willingness to obey authority when it might lead to harming others. Milgram and his students had predicted only 1–3% of participants would administer the maximum shock level. However, in his first official study, 26 of 40 male participants (65%) were convinced to do so and nearly 80% of teachers that continued to administer shocks after 150 volts—the point at which the learner was heard to scream—continued to the maximum of 450 volts. Teachers displayed a range of negative emotional responses to the experiment even as they continued to obey, sometimes pleading with the experimenters to stop the experiment while still participating in it. One teacher believed that he had killed the learner and was moved to tears when he eventually found out that he had not.

Milgram included several variants on the original design of the experiment. In one, the teachers were allowed to select their own voltage levels. In this case, only about 2.5% of participants used the maximum shock level, indicating that they were not inclined to do so without the prompting of an authority figure. In another, there were three teachers, two of whom were not test subjects, but instead had been instructed to protest against the shocks. The existence of peers protesting the experiment made the volunteer teachers less likely to obey. Teachers were also less likely to obey in a variant where they could see the learner and were forced to interact with him.

The Milgram experiment has been highly controversial, both for the ethics of its design and for the reliability of its results and conclusions. It is commonly accepted that the ethics of the experiment would be rejected by mainstream science today, due not only to the handling of the deception involved but also to the extreme stress placed on the teachers, who often reacted emotionally to the experiment and were not debriefed . Some teachers were actually left believing they had genuinely and repeatedly shocked a learner before having the truth revealed to them later. Later researchers examining Milgram’s data also found that the experimenters conducting the tests had sometimes gone off-script in their attempts to coerce the teachers into continuing, and noted that some teachers guessed that they were the subjects of the experiment. However, attempts to validate Milgram’s findings in more ethical ways have often produced similar results.

Meselson and Stahl Experiment

Meselson and Stahl experiment gave the experimental evidence of DNA replication to be semi-conservative type . It was introduced by the Matthew Meselson and Franklin Stahl in the year 1958 . Matthew Meselson and Franklin Stahl have used E.coli as the “ Model organism ” to explain the semiconservative mode of replication.

There are three modes of replication introduced during the 1950s like conservative, semi-conservative and dispersive. The researchers were confused between these three that what could be the actual pattern of DNA replication. In 1958, Matthew Meselson and Franklin Stahl presented their research, where they concluded that the replication of DNA is semiconservative type .

Matthew Meselson and Franklin Stahl have conducted several experiments after the discovery of DNA structure (by the two scientists Watson and Crick ). Watson and Crick’s model is widely accepted to demonstrate the replicative model of DNA. We will discuss the definition, steps and observation of the Meselson and Stahl experiment along with the semi-conservative model of DNA.

Content: Meselson and Stahl Experiment

Semi conservation model of dna, meselson and stahl experiment steps, observation, definition of meselson and stahl experiment.

Meselson and Stahl Experiment gave us the theory of semi-conservative replication of DNA. They have taken E.coli as the model organism and two different isotopes, N-15 and N-14 . The N-15 is the heavier isotope, whereas N-14 is the lighter or common isotope of nitrogen. Meselson and Stahl performed their experiment by first growing the E.coli in the medium containing 15 NH 4 Cl for several generations. They observed that the heavy isotope has incorporated in the genome of E.coli and the cells become more substantial due to 15 N heavy isotope.

Meselson and Stahl then transferred the E.coli cells incorporated with 15 N isotope to the medium containing 14 NH 4 Cl for several generations. After every twenty minutes, the E.coli cells multiply. For the processing of DNA, the cells were centrifuged by the addition of Caesium chloride, resulting  in the formation of the concentration gradient. As a result, light, intermediate and heavy DNA strands will get separated.

After completing their experiment, Meselson and Stahl concluded that after each cell division, half of the DNA would be conserved for every next generation. Therefore, this experiment proves that the DNA replication obeys the semi-conservative mode of replication in which 50% of the DNA conserve for every next generation in a way like 100%, 50%, 25%, and 12.5% and so on.

It is the type of DNA replication. The term semi means “ Half ” and conservative means “ To store ”. The semi-conservative DNA replication results in the two daughter DNAs after the parent DNA replication.

In the two daughter DNA’s, each strand will contain a mixture of the parent DNA’s template strand, and the other with a newly synthesized strand (in F-1 gen ). When the parental DNA replicates, half of the 100%, i.e. 50% of the DNA is conserved by having parent strand and the remaining 50% will produce newly synthesized strands.

After the F-1 gen, the multiplication of the cell will get double, which will produce four DNA strands (in F-2 gen ). In F-2 gen half of 50%, i.e. only 25% of DNA is conserved by having parental strands, and the remaining 75% will produce newly synthesized strands.

• Growth of E.coli : First, the E.coli were grown in the medium containing 15 NH 4 Cl for several generations. NH 4 provides the nitrogen as well as a protein source for the growth of the E.coli. Here, the 15 N is the heavy isotope of nitrogen.
• Incorporation of 15 N : After several generations of E.coli, Meselson and Stahl observed that the 15 N heavy isotope has incorporated between the DNA nucleotides in E.coli.
• Transfer of E.coli cells : The DNA of E.coli labelled with 15 N isotope were transferred to the medium containing 14 NH 4 Cl . Here, the 14 N is the light isotope of nitrogen. The E.coli cells were again allowed to multiply for several generations. The E.coli cells will multiply every 20 minutes for several generations.
• Processing of DNA : For the processing or separation of DNA, the E.coli cells were transferred to the Eppendorf tubes. After that, caesium chloride is added, having a density of 1.71 g/cm 3 (the same of DNA). Finally, the tubes were subjected to high-speed centrifugation 140,000 X g for 20 hours.

The result, after two generations of E.coli, the following results were obtained:

In the F-1 generation : According to the actual observations, two DNA strands (with a mixture of both 15 N and 14 N isotopes) will produce in F-1 gen. The above diagram shows that the semiconservative and dispersive model obeys the pattern of growth explained by Meselson and Stahl.

Thus, it is clear that the DNA does not replicate via “Conservative mode”. According to the conservative model, the DNA replicates to produce one newly synthesized DNA and one parental DNA. Therefore, the conservative model was disapproved, as it does not produce hybrid DNA in the F-1 generation.

In the F-2 generation : According to the actual observation, four DNA strands ( two with hybrid and the remaining two with light DNA ) will produce in the F-2 generation. The hybrid DNA includes a mixture of 15 N and 14 N. The light DNA strands contain a pure 14 N. The diagram shows that only semi-conservative type of replication gave similar results conducted by Meselson and Stahl. Thus, both the conservative and dispersive modes of replication were disapproved.

Therefore, we can conclude that the type of replication in DNA is “ Semi conservative ”. The offsprings have a hybrid DNA containing a mixture of both template and newly synthesized DNA in the semi-conservative model. After each multiplication, the number of offspring will double, and half of the parental DNA will be conserved for the next generation.

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The Milgram Experiment: How Far Will You Go to Obey an Order?

Understand the infamous study and its conclusions about human nature

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A brief Milgram experiment summary is as follows: In the 1960s, psychologist Stanley Milgram conducted a series of studies on the concepts of obedience and authority. His experiments involved instructing study participants to deliver increasingly high-voltage shocks to an actor in another room, who would scream and eventually go silent as the shocks became stronger. The shocks weren't real, but study participants were made to believe that they were.

Today, the Milgram experiment is widely criticized on both ethical and scientific grounds. However, Milgram's conclusions about humanity's willingness to obey authority figures remain influential and well-known.

Key Takeaways: The Milgram Experiment

• The goal of the Milgram experiment was to test the extent of humans' willingness to obey orders from an authority figure.
• Participants were told by an experimenter to administer increasingly powerful electric shocks to another individual. Unbeknownst to the participants, shocks were fake and the individual being shocked was an actor.
• The majority of participants obeyed, even when the individual being shocked screamed in pain.
• The experiment has been widely criticized on ethical and scientific grounds.

Detailed Milgram’s Experiment Summary

In the most well-known version of the Milgram experiment, the 40 male participants were told that the experiment focused on the relationship between punishment, learning, and memory. The experimenter then introduced each participant to a second individual, explaining that this second individual was participating in the study as well. Participants were told that they would be randomly assigned to roles of "teacher" and "learner." However, the "second individual" was an actor hired by the research team, and the study was set up so that the true participant would always be assigned to the "teacher" role.

During the Milgram experiment, the learner was located in a separate room from the teacher (the real participant), but the teacher could hear the learner through the wall. The experimenter told the teacher that the learner would memorize word pairs and instructed the teacher to ask the learner questions. If the learner responded incorrectly to a question, the teacher would be asked to administer an electric shock. The shocks started at a relatively mild level (15 volts) but increased in 15-volt increments up to 450 volts. (In actuality, the shocks were fake, but the participant was led to believe they were real.)

Participants were instructed to give a higher shock to the learner with each wrong answer. When the 150-volt shock was administered, the learner would cry out in pain and ask to leave the study. He would then continue crying out with each shock until the 330-volt level, at which point he would stop responding.

During this process, whenever participants expressed hesitation about continuing with the study, the experimenter would urge them to go on with increasingly firm instructions, culminating in the statement, "You have no other choice, you must go on." The study ended when participants refused to obey the experimenter’s demand, or when they gave the learner the highest level of shock on the machine (450 volts).

Milgram found that participants obeyed the experimenter at an unexpectedly high rate: 65% of the participants gave the learner the 450-volt shock.

Critiques of the Milgram Experiment

The Milgram experiment has been widely criticized on ethical grounds. Milgram’s participants were led to believe that they acted in a way that harmed someone else, an experience that could have had long-term consequences. Moreover, an investigation by writer Gina Perry uncovered that some participants appear to not have been fully debriefed after the study —they were told months later, or not at all, that the shocks were fake and the learner wasn’t harmed. Milgram’s studies could not be perfectly recreated today, because researchers today are required to pay much more attention to the safety and well-being of human research subjects.

Researchers have also questioned the scientific validity of Milgram’s results. In her examination of the study, Perry found that Milgram’s experimenter may have gone off script and told participants to obey many more times than the script specified. Additionally, some research suggests that participants may have figured out that the learner was not harmed: in interviews conducted after the Milgram experiment, some participants reported that they didn’t think the learner was in any real danger. This mindset is likely to have affected their behavior in the study.

Variations on the Milgram Experiment

Milgram and other researchers conducted numerous versions of the experiment over time. The participants' levels of compliance with the experimenter’s demands varied greatly from one study to the next. For example, when participants were in closer proximity to the learner (e.g. in the same room), they were less likely to give the learner the highest level of shock.

Another version of the Milgram experiment brought three "teachers" into the experiment room at once. One was a real participant, and the other two were actors hired by the research team. During the experiment, the two non-participant teachers would quit as the level of shocks began to increase. Milgram found that these conditions made the real participant far more likely to "disobey" the experimenter, too: only 10% of participants gave the 450-volt shock to the learner.

In yet another version of the Milgram experiment, two experimenters were present, and during the experiment, they would begin arguing with one another about whether it was right to continue the study. In this version, none of the participants gave the learner the 450-volt shock.

Replicating the Milgram Experiment

Researchers have sought to replicate Milgram's original study with additional safeguards in place to protect participants. In 2009, Jerry Burger replicated Milgram’s famous experiment at Santa Clara University with new safeguards in place: the highest shock level was 150 volts, and participants were told that the shocks were fake immediately after the experiment ended. Additionally, participants were screened by a clinical psychologist before the experiment began, and those found to be at risk of a negative reaction to the study were deemed ineligible to participate.

Burger found that participants obeyed at similar levels as Milgram’s participants: 82.5% of Milgram’s participants gave the learner the 150-volt shock, and 70% of Burger’s participants did the same.

The Legacy of the Milgram Experiment

Milgram’s interpretation of his research was that everyday people are capable of carrying out unthinkable actions in certain circumstances. His research has been used to explain atrocities such as the Holocaust and the Rwandan genocide, though these applications are by no means widely accepted or agreed upon.

Importantly, not all participants obeyed the experimenter’s demands , and Milgram’s studies shed light on the factors that enable people to stand up to authority. In fact, as sociologist Matthew Hollander writes, we may be able to learn from the participants who disobeyed, as their strategies may enable us to respond more effectively to an unethical situation. The Milgram experiment suggested that human beings are susceptible to obeying authority, but it also demonstrated that obedience is not inevitable.

• Baker, Peter C. “Electric Schlock: Did Stanley Milgram's Famous Obedience Experiments Prove Anything?” Pacific Standard (2013, Sep. 10). https://psmag.com/social-justice/electric-schlock-65377
• Burger, Jerry M. "Replicating Milgram: Would People Still Obey Today?."  American Psychologist 64.1 (2009): 1-11. http://psycnet.apa.org/buy/2008-19206-001
• Gilovich, Thomas, Dacher Keltner, and Richard E. Nisbett. Social Psychology . 1st edition, W.W. Norton & Company, 2006.
• Hollander, Matthew. “How to Be a Hero: Insight From the Milgram Experiment.” HuffPost Contributor Network (2015, Apr. 29). https://www.huffingtonpost.com/entry/how-to-be-a-hero-insight-_b_6566882
• Jarrett, Christian. “New Analysis Suggests Most Milgram Participants Realised the ‘Obedience Experiments’ Were Not Really Dangerous.” The British Psychological Society: Research Digest (2017, Dec. 12). https://digest.bps.org.uk/2017/12/12/interviews-with-milgram-participants-provide-little-support-for-the-contemporary-theory-of-engaged-followership/
• Perry, Gina. “The Shocking Truth of the Notorious Milgram Obedience Experiments.” Discover Magazine Blogs (2013, Oct. 2). http://blogs.discovermagazine.com/crux/2013/10/02/the-shocking-truth-of-the-notorious-milgram-obedience-experiments/
• Romm, Cari. “Rethinking One of Psychology's Most Infamous Experiments.” The Atlantic (2015, Jan. 28) . https://www.theatlantic.com/health/archive/2015/01/rethinking-one-of-psychologys-most-infamous-experiments/384913/
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• What Is Behaviorism in Psychology?
• What Is the Zeigarnik Effect? Definition and Examples
• What Is a Conditioned Response?
• Psychodynamic Theory: Approaches and Proponents
• Social Cognitive Theory: How We Learn From the Behavior of Others
• Kohlberg's Stages of Moral Development
• What's the Difference Between Eudaimonic and Hedonic Happiness?
• Genie Wiley, the Feral Child
• What Is the Law of Effect in Psychology?
• What Is the Recency Effect in Psychology?
• Heuristics: The Psychology of Mental Shortcuts
• What Is Survivor's Guilt? Definition and Examples
• 5 Psychology Studies That Will Make You Feel Good About Humanity
• What Is Cognitive Bias? Definition and Examples

40 Years of Stanford Research Found That People With This One Quality Are More Likely to Succeed

In the 1960s, a Stanford professor named Walter Mischel began conducting a series of important psychological studies.

During his experiments, Mischel and his team tested hundreds of children — most of them around the ages of 4 and 5 years old — and revealed what is now believed to be one of the most important characteristics for success in health, work, and life.

Let’s talk about what happened and, more importantly, how you can use it.

The Marshmallow Experiment

The experiment began by bringing each child into a private room, sitting them down in a chair, and placing a marshmallow on the table in front of them.

At this point, the researcher offered a deal to the child.

The researcher told the child that he was going to leave the room and that if the child did not eat the marshmallow while he was away, then they would be rewarded with a second marshmallow. However, if the child decided to eat the first one before the researcher came back, then they would not get a second marshmallow.

So the choice was simple: one treat right now or two treats later.

The researcher left the room for 15 minutes.

As you can imagine, the footage of the children waiting alone in the room was rather entertaining. Some kids jumped up and ate the first marshmallow as soon as the researcher closed the door. Others wiggled and bounced and scooted in their chairs as they tried to restrain themselves, but eventually gave in to temptation a few minutes later. And finally, a few of the children did manage to wait the entire time.

Published in 1972 , this popular study became known as The Marshmallow Experiment, but it wasn’t the treat that made it famous. The interesting part came years later.

The Power of Delayed Gratification

As the years rolled on and the children grew up, the researchers conducted follow up studies and tracked each child’s progress in a number of areas. What they found was surprising.

The children who were willing to delay gratification and waited to receive the second marshmallow ended up having higher SAT scores, lower levels of substance abuse, lower likelihood of obesity, better responses to stress, better social skills as reported by their parents, and generally better scores in a range of other life measures. (You can see the followup studies here , here , and here .)

The researchers followed each child for more than 40 years and over and over again, the group who waited patiently for the second marshmallow succeed in whatever capacity they were measuring. In other words, this series of experiments proved that the ability to delay gratification was critical for success in life.

And if you look around, you’ll see this playing out everywhere…

• If you delay the gratification of watching television and get your homework done now, then you’ll learn more and get better grades.
• If you delay the gratification of buying desserts and chips at the store, then you’ll eat healthier when you get home.
• If you delay the gratification of finishing your workout early and put in a few more reps, then you’ll be stronger.

… and countless other examples.

Success usually comes down to choosing the pain of discipline over the ease of distraction. And that’s exactly what delayed gratification is all about.

This brings us to an interesting question: Did some children naturally have more self-control, and thus were destined for success? Or can you learn to develop this important trait?

What Determines Your Ability to Delay Gratification?

Researchers at the University of Rochester decided to replicate the marshmallow experiment, but with an important twist. (You can read the study here .)

Before offering the child the marshmallow, the researchers split the children into two groups.

The first group was exposed to a series of unreliable experiences. For example, the researcher gave the child a small box of crayons and promised to bring a bigger one, but never did. Then the researcher gave the child a small sticker and promised to bring a better selection of stickers, but never did.

Meanwhile, the second group had very reliable experiences. They were promised better crayons and got them. They were told about the better stickers and then they received them.

You can imagine the impact these experiences had on the marshmallow test. The children in the unreliable group had no reason to trust that the researchers would bring a second marshmallow and thus they didn’t wait very long to eat the first one.

Meanwhile, the children in the second group were training their brains to see delayed gratification as a positive. Every time the researcher made a promise and then delivered on it, the child’s brain registered two things: 1) waiting for gratification is worth it and 2) I have the capability to wait. As a result, the second group waited an average of four times longer than the first group.

In other words, the child’s ability to delay gratification and display self-control was not a predetermined trait, but rather was impacted by the experiences and environment that surrounded them. In fact, the effects of the environment were almost instantaneous. Just a few minutes of reliable or unreliable experiences were enough to push the actions of each child in one direction or another.

What can you and I learn from all of this?

How to Become Better at Delaying Gratification

Before we go further, let’s clear one thing up: for one reason or another, the Marshmallow Experiment has become particularly popular. You’ll find it mentioned in nearly every major media outlet. But these studies are just one piece of data, a small insight into the story of success. Human behavior (and life in general) is a lot more complex than that, so let’s not pretend that one choice a four-year-old makes will determine the rest of his or her life.

The studies above do make one thing clear: if you want to succeed at something, at some point you will need to find the ability to be disciplined and take action instead of becoming distracted and doing what’s easy. Success in nearly every field requires you to ignore doing something easier (delaying gratification) in favor of doing something harder ( doing the work and putting in your reps ).

But the key takeaway here is that even if you don’t feel like you’re good at delaying gratification now, you can train yourself to become better simply by making a few small improvements. In the case of the children in the study, this meant being exposed to a reliable environment where the researcher promised something and then delivered it.

You and I can do the same thing. We can train our ability to delay gratification, just like we can train our muscles in the gym. And you can do it in the same way as the child and the researcher: by promising something small and then delivering. Over and over again until your brain says, 1) yes, it’s worth it to wait and 2) yes, I have the capability to do this.

Here are 4 simple ways to do exactly that:

• Start incredibly small . Make your new habit “so easy you can’t say no.” (Hat tip to Leo Babauta .)
• Improve one thing, by one percent . Do it again tomorrow.
• Use the “Seinfeld Strategy” to maintain consistency.
• Find a way to get started in less than 2 minutes .

Thanks for reading. You can get more actionable ideas in my popular email newsletter. Each week, I share 3 short ideas from me, 2 quotes from others, and 1 question to think about. Over 3,000,000 people subscribe . Enter your email now and join us.

James Clear writes about habits, decision making, and continuous improvement. He is the author of the #1 New York Times bestseller, Atomic Habits . The book has sold over 20 million copies worldwide and has been translated into more than 60 languages.

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Definition of experiment noun from the Oxford Advanced Learner's Dictionary

• to do/perform/conduct an experiment
• laboratory experiments
• The results of the experiment were inconclusive.
• in an experiment The 30 subjects in each experiment were divided into two groups.
• during an experiment The temperature was kept constant during the experiment.
• experiment on somebody/something Many people do not like the idea of experiments on animals.
• experiment with somebody/something The experiment with cells from other species was unsuccessful.
• by experiment Facts can be established by observation and experiment.
• formulate/​advance a theory/​hypothesis
• build/​construct/​create/​develop a simple/​theoretical/​mathematical model
• develop/​establish/​provide/​use a theoretical/​conceptual framework
• advance/​argue/​develop the thesis that…
• explore an idea/​a concept/​a hypothesis
• make a prediction/​an inference
• base a prediction/​your calculations on something
• investigate/​evaluate/​accept/​challenge/​reject a theory/​hypothesis/​model
• design an experiment/​a questionnaire/​a study/​a test
• do research/​an experiment/​an analysis
• make observations/​measurements/​calculations
• carry out/​conduct/​perform an experiment/​a test/​a longitudinal study/​observations/​clinical trials
• run an experiment/​a simulation/​clinical trials
• repeat an experiment/​a test/​an analysis
• replicate a study/​the results/​the findings
• observe/​study/​examine/​investigate/​assess a pattern/​a process/​a behaviour
• fund/​support the research/​project/​study
• seek/​provide/​get/​secure funding for research
• collect/​gather/​extract data/​information
• yield data/​evidence/​similar findings/​the same results
• analyse/​examine the data/​soil samples/​a specimen
• consider/​compare/​interpret the results/​findings
• fit the data/​model
• confirm/​support/​verify a prediction/​a hypothesis/​the results/​the findings
• prove a conjecture/​hypothesis/​theorem
• draw/​make/​reach the same conclusions
• read/​review the records/​literature
• describe/​report an experiment/​a study
• present/​publish/​summarize the results/​findings
• present/​publish/​read/​review/​cite a paper in a scientific journal
• Further experiments will be carried out to verify this result.
• The results were obtained from four independent experiments.
• proved by experiment
• If the conditions are not right, the experiment will not work.
• In these experiments, chilling is necessary.
• Pavlov's famous experiment with the dog and the dinner bell
• The appropriate concentration of the drug is best determined by experiment.
• NASA carried out experiments to ascertain the effects of weightlessness.
• This pattern emerged clearly in the experiment.
• confirm something
• demonstrate something
• find something
• by experiment
• during an/​the experiment
• in an/​the experiment

Want to learn more?

Find out which words work together and produce more natural-sounding English with the Oxford Collocations Dictionary app. Try it for free as part of the Oxford Advanced Learner’s Dictionary app.

To study people’s roles in prison situations, Zimbardo converted a basement of the Stanford University psychology building into a mock prison.

He advertised asking for volunteers to participate in a study of the psychological effects of prison life.

The 75 applicants who answered the ad were given diagnostic interviews and personality tests to eliminate candidates with psychological problems, medical disabilities, or a history of crime or drug abuse.

24 men judged to be the most physically & mentally stable, the most mature, & the least involved in antisocial behaviors were chosen to participate.

The participants did not know each other prior to the study and were paid $15 per day to take part in the experiment.

Participants were randomly assigned to either the role of prisoner or guard in a simulated prison environment. There were two reserves, and one dropped out, finally leaving ten prisoners and 11 guards.

Prisoners were treated like every other criminal, being arrested at their own homes, without warning, and taken to the local police station. They were fingerprinted, photographed and ‘booked.’

Then they were blindfolded and driven to the psychology department of Stanford University, where Zimbardo had had the basement set out as a prison, with barred doors and windows, bare walls and small cells. Here the deindividuation process began.

When the prisoners arrived at the prison they were stripped naked, deloused, had all their personal possessions removed and locked away, and were given prison clothes and bedding. They were issued a uniform, and referred to by their number only.

The use of ID numbers was a way to make prisoners feel anonymous. Each prisoner had to be called only by his ID number and could only refer to himself and the other prisoners by number.

Their clothes comprised a smock with their number written on it, but no underclothes. They also had a tight nylon cap to cover their hair, and a locked chain around one ankle.

All guards were dressed in identical uniforms of khaki, and they carried a whistle around their neck and a billy club borrowed from the police. Guards also wore special sunglasses, to make eye contact with prisoners impossible.

Three guards worked shifts of eight hours each (the other guards remained on call). Guards were instructed to do whatever they thought was necessary to maintain law and order in the prison and to command the respect of the prisoners. No physical violence was permitted.

Zimbardo observed the behavior of the prisoners and guards (as a researcher), and also acted as a prison warden.

Within a very short time both guards and prisoners were settling into their new roles, with the guards adopting theirs quickly and easily.

Asserting Authority

Within hours of beginning the experiment, some guards began to harass prisoners. At 2:30 A.M. prisoners were awakened from sleep by blasting whistles for the first of many “counts.”

The counts served as a way to familiarize the prisoners with their numbers. More importantly, they provided a regular occasion for the guards to exercise control over the prisoners.

The prisoners soon adopted prisoner-like behavior too. They talked about prison issues a great deal of the time. They ‘told tales’ on each other to the guards.

They started taking the prison rules very seriously, as though they were there for the prisoners’ benefit and infringement would spell disaster for all of them. Some even began siding with the guards against prisoners who did not obey the rules.

Physical Punishment

The prisoners were taunted with insults and petty orders, they were given pointless and boring tasks to accomplish, and they were generally dehumanized.

Push-ups were a common form of physical punishment imposed by the guards. One of the guards stepped on the prisoners” backs while they did push-ups, or made other prisoners sit on the backs of fellow prisoners doing their push-ups.

Asserting Independence

Because the first day passed without incident, the guards were surprised and totally unprepared for the rebellion which broke out on the morning of the second day.

During the second day of the experiment, the prisoners removed their stocking caps, ripped off their numbers, and barricaded themselves inside the cells by putting their beds against the door.

The guards called in reinforcements. The three guards who were waiting on stand-by duty came in and the night shift guards voluntarily remained on duty.

Putting Down the Rebellion

The guards retaliated by using a fire extinguisher which shot a stream of skin-chilling carbon dioxide, and they forced the prisoners away from the doors. Next, the guards broke into each cell, stripped the prisoners naked and took the beds out.

The ringleaders of the prisoner rebellion were placed into solitary confinement. After this, the guards generally began to harass and intimidate the prisoners.

Special Privileges

One of the three cells was designated as a “privilege cell.” The three prisoners least involved in the rebellion were given special privileges. The guards gave them back their uniforms and beds and allowed them to wash their hair and brush their teeth.

Privileged prisoners also got to eat special food in the presence of the other prisoners who had temporarily lost the privilege of eating. The effect was to break the solidarity among prisoners.

Consequences of the Rebellion

Over the next few days, the relationships between the guards and the prisoners changed, with a change in one leading to a change in the other. Remember that the guards were firmly in control and the prisoners were totally dependent on them.

As the prisoners became more dependent, the guards became more derisive towards them. They held the prisoners in contempt and let the prisoners know it. As the guards’ contempt for them grew, the prisoners became more submissive.

As the prisoners became more submissive, the guards became more aggressive and assertive. They demanded ever greater obedience from the prisoners. The prisoners were dependent on the guards for everything, so tried to find ways to please the guards, such as telling tales on fellow prisoners.

Prisoner #8612

Less than 36 hours into the experiment, Prisoner #8612 began suffering from acute emotional disturbance, disorganized thinking, uncontrollable crying, and rage.

After a meeting with the guards where they told him he was weak, but offered him “informant” status, #8612 returned to the other prisoners and said “You can”t leave. You can’t quit.”

Soon #8612 “began to act ‘crazy,’ to scream, to curse, to go into a rage that seemed out of control.” It wasn’t until this point that the psychologists realized they had to let him out.

A Visit from Parents

The next day, the guards held a visiting hour for parents and friends. They were worried that when the parents saw the state of the jail, they might insist on taking their sons home. Guards washed the prisoners, had them clean and polish their cells, fed them a big dinner and played music on the intercom.

After the visit, rumors spread of a mass escape plan. Afraid that they would lose the prisoners, the guards and experimenters tried to enlist help and facilities of the Palo Alto police department.

The guards again escalated the level of harassment, forcing them to do menial, repetitive work such as cleaning toilets with their bare hands.

Catholic Priest

Zimbardo invited a Catholic priest who had been a prison chaplain to evaluate how realistic our prison situation was. Half of the prisoners introduced themselves by their number rather than name.

The chaplain interviewed each prisoner individually. The priest told them the only way they would get out was with the help of a lawyer.

Prisoner #819

Eventually, while talking to the priest, #819 broke down and began to cry hysterically, just like two previously released prisoners had.

The psychologists removed the chain from his foot, the cap off his head, and told him to go and rest in a room that was adjacent to the prison yard. They told him they would get him some food and then take him to see a doctor.

While this was going on, one of the guards lined up the other prisoners and had them chant aloud:

“Prisoner #819 is a bad prisoner. Because of what Prisoner #819 did, my cell is a mess, Mr. Correctional Officer.”

The psychologists realized #819 could hear the chanting and went back into the room where they found him sobbing uncontrollably. The psychologists tried to get him to agree to leave the experiment, but he said he could not leave because the others had labeled him a bad prisoner.

Back to Reality

At that point, Zimbardo said, “Listen, you are not #819. You are [his name], and my name is Dr. Zimbardo. I am a psychologist, not a prison superintendent, and this is not a real prison. This is just an experiment, and those are students, not prisoners, just like you. Let’s go.”

He stopped crying suddenly, looked up and replied, “Okay, let’s go,“ as if nothing had been wrong.

An End to the Experiment

Zimbardo (1973) had intended that the experiment should run for two weeks, but on the sixth day, it was terminated, due to the emotional breakdowns of prisoners, and excessive aggression of the guards.

Christina Maslach, a recent Stanford Ph.D. brought in to conduct interviews with the guards and prisoners, strongly objected when she saw the prisoners being abused by the guards.

Filled with outrage, she said, “It’s terrible what you are doing to these boys!” Out of 50 or more outsiders who had seen our prison, she was the only one who ever questioned its morality.

Zimbardo (2008) later noted, “It wasn’t until much later that I realized how far into my prison role I was at that point — that I was thinking like a prison superintendent rather than a research psychologist.“

This led him to prioritize maintaining the experiment’s structure over the well-being and ethics involved, thereby highlighting the blurring of roles and the profound impact of the situation on human behavior.

Here’s a quote that illustrates how Philip Zimbardo, initially the principal investigator, became deeply immersed in his role as the “Stanford Prison Superintendent (April 19, 2011):

“By the third day, when the second prisoner broke down, I had already slipped into or been transformed into the role of “Stanford Prison Superintendent.” And in that role, I was no longer the principal investigator, worried about ethics. When a prisoner broke down, what was my job? It was to replace him with somebody on our standby list. And that’s what I did. There was a weakness in the study in not separating those two roles. I should only have been the principal investigator, in charge of two graduate students and one undergraduate.”

According to Zimbardo and his colleagues, the Stanford Prison Experiment revealed how people will readily conform to the social roles they are expected to play, especially if the roles are as strongly stereotyped as those of the prison guards.

Because the guards were placed in a position of authority, they began to act in ways they would not usually behave in their normal lives.

The “prison” environment was an important factor in creating the guards’ brutal behavior (none of the participants who acted as guards showed sadistic tendencies before the study).

Therefore, the findings support the situational explanation of behavior rather than the dispositional one.

Zimbardo proposed that two processes can explain the prisoner’s “final submission.”

Deindividuation may explain the behavior of the participants; especially the guards. This is a state when you become so immersed in the norms of the group that you lose your sense of identity and personal responsibility.

The guards may have been so sadistic because they did not feel what happened was down to them personally – it was a group norm. They also may have lost their sense of personal identity because of the uniform they wore.

Also, learned helplessness could explain the prisoner’s submission to the guards. The prisoners learned that whatever they did had little effect on what happened to them. In the mock prison the unpredictable decisions of the guards led the prisoners to give up responding.

After the prison experiment was terminated, Zimbardo interviewed the participants. Here’s an excerpt:

‘Most of the participants said they had felt involved and committed. The research had felt “real” to them. One guard said, “I was surprised at myself. I made them call each other names and clean the toilets out with their bare hands. I practically considered the prisoners cattle and I kept thinking I had to watch out for them in case they tried something.” Another guard said “Acting authoritatively can be fun. Power can be a great pleasure.” And another: “… during the inspection I went to Cell Two to mess up a bed which a prisoner had just made and he grabbed me, screaming that he had just made it and that he was not going to let me mess it up. He grabbed me by the throat and although he was laughing I was pretty scared. I lashed out with my stick and hit him on the chin although not very hard, and when I freed myself I became angry.”’

Most of the guards found it difficult to believe that they had behaved in the brutal ways that they had. Many said they hadn’t known this side of them existed or that they were capable of such things.

The prisoners, too, couldn’t believe that they had responded in the submissive, cowering, dependent way they had. Several claimed to be assertive types normally.

When asked about the guards, they described the usual three stereotypes that can be found in any prison: some guards were good, some were tough but fair, and some were cruel.

A further explanation for the behavior of the participants can be described in terms of reinforcement.  The escalation of aggression and abuse by the guards could be seen as being due to the positive reinforcement they received both from fellow guards and intrinsically in terms of how good it made them feel to have so much power.

Similarly, the prisoners could have learned through negative reinforcement that if they kept their heads down and did as they were told, they could avoid further unpleasant experiences.

Critical Evaluation

Ecological validity.

The Stanford Prison Experiment is criticized for lacking ecological validity in its attempt to simulate a real prison environment. Specifically, the “prison” was merely a setup in the basement of Stanford University’s psychology department.

The student “guards” lacked professional training, and the experiment’s duration was much shorter than real prison sentences. Furthermore, the participants, who were college students, didn’t reflect the diverse backgrounds typically found in actual prisons in terms of ethnicity, education, and socioeconomic status.

None had prior prison experience, and they were chosen due to their mental stability and low antisocial tendencies. Additionally, the mock prison lacked spaces for exercise or rehabilitative activities.

Demand characteristics

Demand characteristics could explain the findings of the study. Most of the guards later claimed they were simply acting. Because the guards and prisoners were playing a role, their behavior may not be influenced by the same factors which affect behavior in real life. This means the study’s findings cannot be reasonably generalized to real life, such as prison settings. I.e, the study has low ecological validity.

One of the biggest criticisms is that strong demand characteristics confounded the study. Banuazizi and Movahedi (1975) found that the majority of respondents, when given a description of the study, were able to guess the hypothesis and predict how participants were expected to behave.

This suggests participants may have simply been playing out expected roles rather than genuinely conforming to their assigned identities.

In addition, revelations by Zimbardo (2007) indicate he actively encouraged the guards to be cruel and oppressive in his orientation instructions prior to the start of the study. For example, telling them “they [the prisoners] will be able to do nothing and say nothing that we don’t permit.”

He also tacitly approved of abusive behaviors as the study progressed. This deliberate cueing of how participants should act, rather than allowing behavior to unfold naturally, indicates the study findings were likely a result of strong demand characteristics rather than insightful revelations about human behavior.

However, there is considerable evidence that the participants did react to the situation as though it was real. For example, 90% of the prisoners’ private conversations, which were monitored by the researchers, were on the prison conditions, and only 10% of the time were their conversations about life outside of the prison.

The guards, too, rarely exchanged personal information during their relaxation breaks – they either talked about ‘problem prisoners,’ other prison topics, or did not talk at all. The guards were always on time and even worked overtime for no extra pay.

When the prisoners were introduced to a priest, they referred to themselves by their prison number, rather than their first name. Some even asked him to get a lawyer to help get them out.

Fourteen years after his experience as prisoner 8612 in the Stanford Prison Experiment, Douglas Korpi, now a prison psychologist, reflected on his time and stated (Musen and Zimbardo 1992):

“The Stanford Prison Experiment was a very benign prison situation and it promotes everything a normal prison promotes — the guard role promotes sadism, the prisoner role promotes confusion and shame”.

Sample bias

The study may also lack population validity as the sample comprised US male students. The study’s findings cannot be applied to female prisons or those from other countries. For example, America is an individualist culture (where people are generally less conforming), and the results may be different in collectivist cultures (such as Asian countries).

Carnahan and McFarland (2007) have questioned whether self-selection may have influenced the results – i.e., did certain personality traits or dispositions lead some individuals to volunteer for a study of “prison life” in the first place?

All participants completed personality measures assessing: aggression, authoritarianism, Machiavellianism, narcissism, social dominance, empathy, and altruism. Participants also answered questions on mental health and criminal history to screen out any issues as per the original SPE.

Results showed that volunteers for the prison study, compared to the control group, scored significantly higher on aggressiveness, authoritarianism, Machiavellianism, narcissism, and social dominance. They scored significantly lower on empathy and altruism.

A follow-up role-playing study found that self-presentation biases could not explain these differences. Overall, the findings suggest that volunteering for the prison study was influenced by personality traits associated with abusive tendencies.

Zimbardo’s conclusion may be wrong

While implications for the original SPE are speculative, this lends support to a person-situation interactionist perspective, rather than a purely situational account.

It implies that certain individuals are drawn to and selected into situations that fit their personality, and that group composition can shape behavior through mutual reinforcement.

Contributions to psychology

Another strength of the study is that the harmful treatment of participants led to the formal recognition of ethical  guidelines by the American Psychological Association. Studies must now undergo an extensive review by an institutional review board (US) or ethics committee (UK) before they are implemented.

Most institutions, such as universities, hospitals, and government agencies, require a review of research plans by a panel. These boards review whether the potential benefits of the research are justifiable in light of the possible risk of physical or psychological harm.

These boards may request researchers make changes to the study’s design or procedure, or, in extreme cases, deny approval of the study altogether.

Contribution to prison policy

A strength of the study is that it has altered the way US prisons are run. For example, juveniles accused of federal crimes are no longer housed before trial with adult prisoners (due to the risk of violence against them).

However, in the 25 years since the SPE, U.S. prison policy has transformed in ways counter to SPE insights (Haney & Zimbardo, 1995):

• Rehabilitation was abandoned in favor of punishment and containment. Prison is now seen as inflicting pain rather than enabling productive re-entry.
• Sentencing became rigid rather than accounting for inmates’ individual contexts. Mandatory minimums and “three strikes” laws over-incarcerate nonviolent crimes.
• Prison construction boomed, and populations soared, disproportionately affecting minorities. From 1925 to 1975, incarceration rates held steady at around 100 per 100,000. By 1995, rates tripled to over 600 per 100,000.
• Drug offenses account for an increasing proportion of prisoners. Nonviolent drug offenses make up a large share of the increased incarceration.
• Psychological perspectives have been ignored in policymaking. Legislators overlooked insights from social psychology on the power of contexts in shaping behavior.
• Oversight retreated, with courts deferring to prison officials and ending meaningful scrutiny of conditions. Standards like “evolving decency” gave way to “legitimate” pain.
• Supermax prisons proliferated, isolating prisoners in psychological trauma-inducing conditions.

The authors argue psychologists should reengage to:

• Limit the use of imprisonment and adopt humane alternatives based on the harmful effects of prison environments
• Assess prisons’ total environments, not just individual conditions, given situational forces interact
• Prepare inmates for release by transforming criminogenic post-release contexts
• Address socioeconomic risk factors, not just incarcerate individuals
• Develop contextual prediction models vs. focusing only on static traits
• Scrutinize prison systems independently, not just defer to officials shaped by those environments
• Generate creative, evidence-based reforms to counter over-punitive policies

Psychology once contributed to a more humane system and can again counter the U.S. “rage to punish” with contextual insights (Haney & Zimbardo, 1998).

Evidence for situational factors

Zimbardo (1995) further demonstrates the power of situations to elicit evil actions from ordinary, educated people who likely would never have done such things otherwise. It was another situation-induced “transformation of human character.”

• Unit 731 was a covert biological and chemical warfare research unit of the Japanese army during WWII.
• It was led by General Shiro Ishii and involved thousands of doctors and researchers.
• Unit 731 set up facilities near Harbin, China to conduct lethal human experimentation on prisoners, including Allied POWs.
• Experiments involved exposing prisoners to things like plague, anthrax, mustard gas, and bullets to test biological weapons. They infected prisoners with diseases and monitored their deaths.
• At least 3,000 prisoners died from these brutal experiments. Many were killed and dissected.
• The doctors in Unit 731 obeyed orders unquestioningly and conducted these experiments in the name of “medical science.”
• After the war, the vast majority of doctors who participated faced no punishment and went on to have prestigious careers. This was largely covered up by the U.S. in exchange for data.
• It shows how normal, intelligent professionals can be led by situational forces to systematically dehumanize victims and conduct incredibly cruel and lethal experiments on people.
• Even healers trained to preserve life used their expertise to destroy lives when the situational forces compelled obedience, nationalism, and wartime enmity.

Evidence for an interactionist approach

The results are also relevant for explaining abuses by American guards at Abu Ghraib prison in Iraq.

An interactionist perspective recognizes that volunteering for roles as prison guards attracts those already prone to abusive tendencies, which are intensified by the prison context.

This counters a solely situationist view of good people succumbing to evil situational forces.

Ethical Issues

The study has received many ethical criticisms, including lack of fully informed consent by participants as Zimbardo himself did not know what would happen in the experiment (it was unpredictable). Also, the prisoners did not consent to being “arrested” at home. The prisoners were not told partly because final approval from the police wasn’t given until minutes before the participants decided to participate, and partly because the researchers wanted the arrests to come as a surprise. However, this was a breach of the ethics of Zimbardo’s own contract that all of the participants had signed.

Protection of Participants

Participants playing the role of prisoners were not protected from psychological harm, experiencing incidents of humiliation and distress. For example, one prisoner had to be released after 36 hours because of uncontrollable bursts of screaming, crying, and anger.

Here’s a quote from Philip G. Zimbardo, taken from an interview on the Stanford Prison Experiment’s 40th anniversary (April 19, 2011):

“In the Stanford prison study, people were stressed, day and night, for 5 days, 24 hours a day. There’s no question that it was a high level of stress because five of the boys had emotional breakdowns, the first within 36 hours. Other boys that didn’t have emotional breakdowns were blindly obedient to corrupt authority by the guards and did terrible things to each other. And so it is no question that that was unethical. You can’t do research where you allow people to suffer at that level.”

“After the first one broke down, we didn’t believe it. We thought he was faking. There was actually a rumor he was faking to get out. He was going to bring his friends in to liberate the prison. And/or we believed our screening procedure was inadequate, [we believed] that he had some mental defect that we did not pick up. At that point, by the third day, when the second prisoner broke down, I had already slipped into or been transformed into the role of “Stanford Prison Superintendent.” And in that role, I was no longer the principal investigator, worried about ethics.”

However, in Zimbardo’s defense, the emotional distress experienced by the prisoners could not have been predicted from the outset.

Approval for the study was given by the Office of Naval Research, the Psychology Department, and the University Committee of Human Experimentation.

This Committee also did not anticipate the prisoners’ extreme reactions that were to follow. Alternative methodologies were looked at that would cause less distress to the participants but at the same time give the desired information, but nothing suitable could be found.

Withdrawal 

Although guards were explicitly instructed not to physically harm prisoners at the beginning of the Stanford Prison Experiment, they were allowed to induce feelings of boredom, frustration, arbitrariness, and powerlessness among the inmates.

This created a pervasive atmosphere where prisoners genuinely believed and even reinforced among each other, that they couldn’t leave the experiment until their “sentence” was completed, mirroring the inescapability of a real prison.

Even though two participants (8612 and 819) were released early, the impact of the environment was so profound that prisoner 416, reflecting on the experience two months later, described it as a “prison run by psychologists rather than by the state.”

Extensive group and individual debriefing sessions were held, and all participants returned post-experimental questionnaires several weeks, then several months later, and then at yearly intervals. Zimbardo concluded there were no lasting negative effects.

Zimbardo also strongly argues that the benefits gained from our understanding of human behavior and how we can improve society should outbalance the distress caused by the study.

However, it has been suggested that the US Navy was not so much interested in making prisons more human and were, in fact, more interested in using the study to train people in the armed services to cope with the stresses of captivity.

Discussion Questions

What are the effects of living in an environment with no clocks, no view of the outside world, and minimal sensory stimulation?

Consider the psychological consequences of stripping, delousing, and shaving the heads of prisoners or members of the military. Whattransformations take place when people go through an experience like this?

The prisoners could have left at any time, and yet, they didn’t. Why?

After the study, how do you think the prisoners and guards felt?

If you were the experimenter in charge, would you have done this study? Would you have terminated it earlier? Would you have conducted a follow-up study?

Frequently Asked Questions

What happened to prisoner 8612 after the experiment.

Douglas Korpi, as prisoner 8612, was the first to show signs of severe distress and demanded to be released from the experiment. He was released on the second day, and his reaction to the simulated prison environment highlighted the study’s ethical issues and the potential harm inflicted on participants.

After the experiment, Douglas Korpi graduated from Stanford University and earned a Ph.D. in clinical psychology. He pursued a career as a psychotherapist, helping others with their mental health struggles.

Why did Zimbardo not stop the experiment?

Zimbardo did not initially stop the experiment because he became too immersed in his dual role as the principal investigator and the prison superintendent, causing him to overlook the escalating abuse and distress among participants.

It was only after an external observer, Christina Maslach, raised concerns about the participants’ well-being that Zimbardo terminated the study.

What happened to the guards in the Stanford Prison Experiment?

In the Stanford Prison Experiment, the guards exhibited abusive and authoritarian behavior, using psychological manipulation, humiliation, and control tactics to assert dominance over the prisoners. This ultimately led to the study’s early termination due to ethical concerns.

What did Zimbardo want to find out?

Zimbardo aimed to investigate the impact of situational factors and power dynamics on human behavior, specifically how individuals would conform to the roles of prisoners and guards in a simulated prison environment.

He wanted to explore whether the behavior displayed in prisons was due to the inherent personalities of prisoners and guards or the result of the social structure and environment of the prison itself.

What were the results of the Stanford Prison Experiment?

The results of the Stanford Prison Experiment showed that situational factors and power dynamics played a significant role in shaping participants’ behavior. The guards became abusive and authoritarian, while the prisoners became submissive and emotionally distressed.

The experiment revealed how quickly ordinary individuals could adopt and internalize harmful behaviors due to their assigned roles and the environment.

Banuazizi, A., & Movahedi, S. (1975). Interpersonal dynamics in a simulated prison: A methodological analysis. American Psychologist, 30 , 152-160.

Carnahan, T., & McFarland, S. (2007). Revisiting the Stanford prison experiment: Could participant self-selection have led to the cruelty? Personality and Social Psychology Bulletin, 33, 603-614.

Drury, S., Hutchens, S. A., Shuttlesworth, D. E., & White, C. L. (2012). Philip G. Zimbardo on his career and the Stanford Prison Experiment’s 40th anniversary.  History of Psychology ,  15 (2), 161.

Griggs, R. A., & Whitehead, G. I., III. (2014). Coverage of the Stanford Prison Experiment in introductory social psychology textbooks. Teaching of Psychology, 41 , 318 –324.

Haney, C., Banks, W. C., & Zimbardo, P. G. (1973). A study of prisoners and guards in a simulated prison . Naval Research Review , 30, 4-17.

Haney, C., & Zimbardo, P. (1998). The past and future of U.S. prison policy: Twenty-five years after the Stanford Prison Experiment.  American Psychologist, 53 (7), 709–727.

Musen, K. & Zimbardo, P. (1992) (DVD) Quiet Rage: The Stanford Prison Experiment Documentary.

Zimbardo, P. G. (Consultant, On-Screen Performer), Goldstein, L. (Producer), & Utley, G. (Correspondent). (1971, November 26). Prisoner 819 did a bad thing: The Stanford Prison Experiment [Television series episode]. In L. Goldstein (Producer), Chronolog. New York, NY: NBC-TV.

Zimbardo, P. G. (1973). On the ethics of intervention in human psychological research: With special reference to the Stanford prison experiment.  Cognition ,  2 (2), 243-256.

Zimbardo, P. G. (1995). The psychology of evil: A situationist perspective on recruiting good people to engage in anti-social acts.  Japanese Journal of Social Psychology ,  11 (2), 125-133.

Zimbardo, P.G. (2007). The Lucifer effect: Understanding how good people turn evil . New York, NY: Random House.

Further Information

• Reicher, S., & Haslam, S. A. (2006). Rethinking the psychology of tyranny: The BBC prison study. The British Journal of Social Psychology, 45 , 1.
• Coverage of the Stanford Prison Experiment in introductory psychology textbooks
• The Stanford Prison Experiment Official Website