Famous Experiments
Landmark experiments that shaped our understanding of the human mind — from Pavlov's dogs to the Stanford Prison.
80 experiments
- Serial Position Effect
We remember the first (primacy effect) and last (recency effect) items of a list best. Middle items are lost. One of the foundational discoveries of memory psychology.
- Spacing Effect
Learning distributed over time is far more effective than massed study. One hour per day for 7 days beats 7 hours straight. Founded the basis for spaced repetition.
- Müller-Lyer Illusion
Two lines of equal length appear different when one has inward-pointing arrows and the other outward. One of the most famous visual illusions, proving perception is not a copy of reality.
- Pavlov's Classical Conditioning
Pavlov discovered that dogs could learn to salivate in response to a neutral stimulus (a bell) if it was repeatedly presented before food. Established the foundations of behaviorism.
- Triplett's Social Facilitation
Cyclists raced faster in competition than alone. Children wound fishing reels faster with others present. The first social psychology experiment: others' presence improves performance on simple tasks.
- Ringelmann Effect
When pulling a rope in a group, each person exerts less effort than alone. Individual productivity drops as group size increases (social loafing).
- Little Albert Experiment
A 9-month-old was conditioned to fear a white rat by pairing it with a loud noise. The fear generalized to other furry objects. Demonstrated that phobias can be learned.
- Halo Effect
Thorndike found that military officers who rated a soldier as attractive also attributed intelligence and leadership to him. One positive quality radiates to all others.
- Hawthorne Effect
Factory workers increased productivity when observed, regardless of environmental changes. The mere fact of being observed modifies behavior.
- Köhler's Insight Learning
Chimpanzees stacked boxes to reach bananas on the ceiling without trial and error. They had a "eureka moment." Proved learning isn't always gradual — it can be sudden insight.
- Zeigarnik Effect
Participants remembered interrupted tasks better than completed ones. The mind keeps unfinished tasks active, creating psychic tension until completion. That's why TV shows end on cliffhangers.
- Stroop Effect
Naming the ink color of a word is slower when the word spells a different color ("RED" in blue ink). Demonstrates interference between automatic and controlled processes.
- Sherif's Autokinetic Effect
In a dark room, a stationary point of light appears to move. When people estimate movement in a group, their answers converge. Social norms emerge spontaneously from interaction.
- Skinner Box
Rats and pigeons in conditioning chambers learned to press levers for rewards. Demonstrated operant conditioning: behavior is shaped by its consequences.
- Clark Doll Study
African-American children preferred white dolls and attributed positive qualities to white, negative to black. Showed how segregation damaged racial self-esteem. Was key evidence in Brown v. Board of Education.
- Asch Conformity Experiment
Participants compared line lengths. When confederates gave wrong answers, 75% of participants conformed at least once. Demonstrated group pressure on individual judgment.
- Piaget's Conservation Tasks
Young children don't understand that quantity is conserved when shape changes (water in differently shaped glasses). Demonstrated stages of cognitive development.
- Hovland Persuasion Experiments
Hovland studied how source credibility, argument order, and emotions influence persuasion. Established the scientific foundations of persuasive communication.
- Cocktail Party Effect
We can focus on one conversation in a noisy room and detect our name in another we weren't listening to. Demonstrates selective attention and unconscious processing.
- Robbers Cave Experiment
Two groups of boys at a camp rapidly developed intergroup hostility. Only superordinate goals reduced conflict. Demonstrated how intergroup prejudices form and resolve.
- Placebo Effect
Wounded soldiers improved with saline injections they believed were morphine. The expectation of cure activates real pain relief mechanisms in the brain.
- Festinger's Doomsday Cult Infiltration
Festinger infiltrated a group predicting the end of the world. When the prophecy failed, members strengthened their beliefs. Originated the theory of cognitive dissonance.
- Harlow's Monkeys
Infant macaques separated from mothers preferred a warm cloth surrogate without food over a wire surrogate with a bottle. Proved that attachment and contact are more important than feeding.
- Broadbent's Dichotic Listening
Participants with different messages in each ear could only process one. Established the filter model of attention: the brain selects one channel and discards the other.
- Kohlberg's Heinz Dilemma
Should a man steal a drug to save his wife? The answer doesn't matter — the reasoning does. Kohlberg identified 6 stages of moral development, from obedience to universal principles.
- Cognitive Dissonance Experiment
Participants paid little ($1) to lie about a boring task came to believe it; those paid a lot ($20) didn't. Proved that the mind adjusts beliefs to reduce discomfort.
- Visual Cliff
Crawling infants refused to cross the transparent section of a glass platform, proving that depth perception is present from 6 months of age.
- Split-Brain Experiments
Patients with severed corpus callosum showed each hemisphere processes information independently. The left dominates language and the right spatial tasks.
- Bobo Doll Experiment
Children who watched adults hit an inflatable doll reproduced the same aggression. Demonstrated social learning: children imitate observed violence.
- Milgram Obedience Experiment
Participants administered (fake) electric shocks to an actor when ordered by an authority figure. 65% went to maximum voltage, demonstrating the power of obedience to authority.
- Garcia's Taste Aversion
Rats that became ill after eating a novel food avoided it forever, even if illness was caused by radiation hours later. Proved that learning doesn't require immediacy.
- Foot-in-the-Door Technique
Agreeing to a small favor increases the likelihood of agreeing to a large one later. Commitment to a small action changes our self-perception and makes us say yes to the next.
- Just-World Hypothesis
Observers blamed victims of electric shocks they couldn't avoid. We believe people deserve what happens to them because we need to believe the world is just.
- Wason Selection Task
Only 10% of participants chose the correct cards to verify a logical rule. Demonstrates confirmation bias: we seek evidence confirming our beliefs, not disconfirming them.
- Learned Helplessness
Dogs that couldn't escape electric shocks stopped trying even when given an exit. Demonstrated that helplessness can be learned — a model for depression.
- Trolley Problem
Would you divert a trolley to save 5 and kill 1? What about pushing someone? Most say yes to the first and no to the second, revealing the difference between action and moral omission.
- Weapons Effect
Angered participants administered more shocks when a gun was on the table than a badminton racket. The mere presence of weapons increases aggression.
- Bystander Effect
The more witnesses to an emergency, the less likely anyone is to help. Demonstrated the diffusion of responsibility in emergency situations.
- Blue Eyes / Brown Eyes Exercise
A teacher divided her class by eye color, declaring one group superior. Within hours, the dominant group became aggressive and the inferior group submissive. Showed how discrimination is created.
- Mere Exposure Effect
People prefer stimuli they've seen before, even without remembering them. Familiarity breeds preference. Explains why we like songs more after hearing them several times.
- Pygmalion Effect (Rosenthal)
Teachers (falsely) told certain students were gifted actually caused those students to improve. Others' expectations become self-fulfilling prophecies.
- Benjamin Franklin Effect
Participants who did a favor for the experimenter rated him more positively afterward. Doing someone a favor makes us like them more, not less. Action changes attitude.
- Zimbardo's Deindividuation
Hooded participants administered longer shocks than identifiable ones. Anonymity reduces self-awareness and increases aggressive and antisocial behavior.
- Ainsworth's Strange Situation
Children aged 12–18 months were observed during brief separations from mother. Identified three attachment styles: secure, anxious-resistant, and avoidant, revolutionizing attachment theory.
- Mirror Self-Recognition Test
Chimpanzees with a mark on their forehead tried to remove it when seeing their reflection, demonstrating self-awareness. Human infants achieve this at 18 months. A foundational test of self-consciousness.
- Stanford Prison Experiment
Students were randomly assigned as guards or prisoners. Within days, guards became abusive and prisoners became depressed. Revealed how social roles transform behavior.
- Minimal Group Paradigm
Dividing people into groups by trivial criteria (preference for Klee or Kandinsky) already generated in-group favoritism. Group identity and discrimination arise from almost nothing.
- Actor-Observer Bias
We explain our own behavior by situation ("I was tired") but others' by character ("they're lazy"). Perspective (actor vs observer) radically changes causal attributions.
- Reciprocity Norm
Participants who received a free Coca-Cola from a stranger bought twice as many raffle tickets as those who didn't. A small favor creates a disproportionately large obligation to reciprocate.
- Marshmallow Test
4-year-olds chose between one treat now or two if they waited 15 minutes. Those who waited showed better academic and social outcomes years later. Demonstrated the power of impulse control.
- Mischel's Delay of Gratification
Children who used distraction strategies (thinking fun thoughts, covering the treat) waited much longer. Showed that self-control is a learnable skill, not a fixed trait.
- Rosenhan Experiment
Sane people faked hearing voices to gain admission to psychiatric hospitals. None were detected as impostors. Challenged the reliability of psychiatric diagnosis.
- Good Samaritan Experiment
Seminary students rushing to give a talk on the Good Samaritan didn't help a fallen man. Situation (being in a hurry) predicts helping better than personality.
- Availability Heuristic
We think plane crashes are more common than diabetes deaths because we remember them better. We judge event probability by the ease with which we recall examples.
- Anchoring Effect
Spinning a wheel of fortune before estimating the percentage of African countries in the UN influenced answers. Arbitrary initial numbers anchor our subsequent judgments.
- Misinformation Effect
After seeing a car accident, asking "How fast were they going when they smashed?" vs "...contacted?" changed memories. Leading questions alter memory.
- Flow State Research
Csikszentmihalyi interviewed thousands about their peak engagement moments. Identified flow state: when challenge matches skill, we experience total absorption.
- Door-in-the-Face Technique
Asking for a huge favor first (rejected) then a small one increases compliance with the second. The perceived concession generates reciprocity. One of the most effective persuasion techniques.
- McGurk Effect
Seeing a person mouth "ga" while hearing "ba" makes us perceive "da." The brain fuses visual and auditory information, creating a perception that exists in neither source.
- Fundamental Attribution Error
Participants attributed an essayist's opinions to their character even knowing the topic was assigned. We tend to attribute others' behavior to character rather than situation.
- Depressive Realism
People with mild depression estimated their control over outcomes more accurately than non-depressed people. Healthy people have positive illusions; depressives see reality more clearly.
- Social Identity Theory
Our self-esteem partly depends on the prestige of our groups. To maintain it, we favor the in-group and denigrate the out-group, even without objective reasons.
- Prospect Theory (Loss Aversion)
Losing €100 hurts more than gaining €100 feels good. People are risk-averse for gains but risk-seeking for losses. Losses weigh roughly twice as much as equivalent gains.
- Framing Effect
"Save 200 out of 600" or "400 will die" are the same, but people prefer the first. How information is framed radically changes our decisions.
- Terror Management Theory
Reminding participants of their mortality makes them defend their cultural beliefs more strongly and reject those who are different. Death awareness drives much of human social behavior.
- Adult Attachment Styles
Hazan and Shaver showed that childhood attachment patterns (secure, anxious, avoidant) reproduce in adult romantic relationships. Romantic love is an attachment process.
- Facial Feedback Hypothesis
Participants holding a pen in their teeth (forcing a smile) found cartoons funnier than those holding it with lips. Facial expression influences emotion.
- Endowment Effect
Participants who received a mug valued it at twice the price of those who could buy it. We value what we already own more simply because it's ours.
- Peak-End Rule
Patients with longer but less painful colonoscopies rated them as less unpleasant. We remember experiences by their peak intensity and ending, not duration.
- False Memory Experiment
Loftus implanted false memories (being lost in a mall as a child) in 25% of participants. Proved that memory is not a faithful recording but a malleable reconstruction.
- Stereotype Threat
African-American students performed worse on a test when told it measured intelligence. Activating a negative stereotype about one's own group impairs performance.
- Change Blindness
An experimenter asked a pedestrian for directions; during a brief interruption, was replaced by another person. 50% didn't notice. We don't see changes we don't expect.
- Ego Depletion
Participants who resisted chocolate cookies to eat radishes gave up sooner on an impossible puzzle. Self-control uses a limited resource that depletes with use.
- Implicit Association Test (IAT)
By measuring response speed in linking concepts, the IAT reveals implicit biases people don't consciously recognize. Showed that prejudices operate outside awareness.
- Rubber Hand Illusion
By simultaneously stroking a visible rubber hand and the hidden real hand, participants feel the rubber hand is theirs. Shows that body ownership is a brain construction, not a fixed fact.
- Invisible Gorilla
Participants counting ball passes didn't see a person in a gorilla suit. Demonstrated inattentional blindness: when focused, we're blind to the unexpected.
- Dunning-Kruger Effect
Incompetent people overestimate their abilities while competent people underestimate theirs. Incompetence prevents recognizing one's own incompetence.
- Zimbardo's Time Perspective
Zimbardo showed that time orientation (past, present, future) predicts health behaviors, academic success, and life satisfaction. A balance of hedonistic present and future is key.
- Spotlight Effect
Participants wearing embarrassing t-shirts overestimated how many people noticed. We believe others observe us far more than they actually do.
- Dead Salmon fMRI
A dead Atlantic salmon was placed in an fMRI scanner and shown photographs of people in social situations. Analysis without correction for multiple comparisons detected "significant brain activity" in the dead fish, demonstrating the danger of false positives in neuroimaging and the need for rigorous statistical correction.