Psychology is built on groundbreaking experiments that reveal the complexities of behavior, cognition, perception, and social dynamics. While no single source lists exactly 100 “most interesting” experiments, I’ve compiled this list from influential studies across classic and modern psychology. These include ethical landmarks, controversial trials, and insightful demonstrations that continue to influence the field. Many early experiments would not meet today’s ethical standards, but they provided foundational insights. I’ve numbered them, with brief descriptions of the setup, key findings, and implications.
- Pavlov’s Classical Conditioning (1897): Ivan Pavlov rang a bell before feeding dogs, conditioning them to salivate at the bell alone. This demonstrated how neutral stimuli can trigger learned responses, forming the basis of behaviorism.
- Little Albert Experiment (1920): John Watson and Rosalie Rayner conditioned a baby (“Albert”) to fear a white rat by pairing it with loud noises. It showed fears can be learned but raised ethical concerns about trauma.
- Hawthorne Effect (1924-1932): Workers at the Hawthorne plant increased productivity when observed, regardless of changes like lighting. This highlighted how awareness of being studied affects behavior.
- Thorndike’s Halo Effect (1920): Edward Thorndike found that ratings of one positive trait (e.g., intelligence) influence perceptions of unrelated traits. It explains cognitive biases in judgments.
- Asch Conformity Experiments (1951): Solomon Asch had participants match line lengths; confederates gave wrong answers, leading 75% of real participants to conform at least once. It revealed group pressure’s power.
- Skinner’s Operant Conditioning Chamber (1930s): B.F. Skinner used “Skinner boxes” to reward/punish rats or pigeons for behaviors like lever-pressing. This shaped reinforcement theories in learning and addiction.
- Milgram Obedience Study (1961): Participants “shocked” a learner (actor) under authority’s orders, with 65% going to 450 volts. It showed obedience to authority can override morals.
- Harlow’s Monkey Experiments (1950s): Baby rhesus monkeys preferred cloth “mothers” for comfort over wire ones for food. This emphasized emotional bonds over mere nourishment in attachment.
- Robbers Cave Experiment (1954): Muzafer Sherif divided boys into rival camps, sparking conflict, then resolved it via superordinate goals. It illustrated realistic conflict theory and cooperation.
- Bobo Doll Experiment (1961): Albert Bandura showed children imitating adult aggression toward a doll after observing it. This supported social learning theory over innate aggression.
- Stanford Prison Experiment (1971): Philip Zimbardo assigned students as guards/prisoners; abuse emerged in days. It demonstrated situational roles’ impact on behavior (ended early for ethics).
- Marshmallow Test (1972): Walter Mischel offered kids one marshmallow now or two later; delayers had better life outcomes. It linked self-control to success.
- Visual Cliff (1960): Eleanor Gibson and Richard Walk tested infants on a glass “cliff”; most avoided the drop side. This suggested depth perception is innate or quickly learned.
- Rosenhan Experiment (1973): David Rosenhan’s team faked hallucinations to enter psychiatric hospitals; once admitted, normal behavior was ignored. It critiqued diagnostic reliability.
- Learned Helplessness (1965): Martin Seligman shocked dogs inescapably, leading them to stop trying even when escape was possible. This modeled depression and resilience.
- Monster Study (1939): Wendell Johnson induced stuttering in orphans via criticism. It showed negative feedback’s harm but was unethical and hidden for years.
- Blue Eyes/Brown Eyes Exercise (1968): Jane Elliott divided third-graders by eye color, discriminating one group. It vividly taught prejudice’s effects the day after MLK’s assassination.
- Fantz’s Infant Perception (1961): Robert Fantz showed babies prefer patterned faces, proving early visual preferences and informing infant cognition studies.
- Magical Number Seven (1956): George Miller found short-term memory holds 7±2 items. This influenced cognitive psychology and chunking strategies.
- Bystander Effect (1968): After Kitty Genovese’s murder, John Darley and Bibb Latané staged emergencies; help decreased with more bystanders due to diffusion of responsibility.
- Violinist in the Metro (2007): Joshua Bell played incognito in a D.C. subway; few stopped. It explored context’s role in perception and attention.
- Cognitive Dissonance (1957): Leon Festinger had participants do boring tasks for low pay; they rated tasks higher to reduce dissonance. This explained attitude-behavior inconsistencies.
- False Memory Implant (1990s): Elizabeth Loftus planted memories of being lost in a mall. It showed memories’ malleability, impacting eyewitness testimony.
- Three Christs of Ypsilanti (1959): Milton Rokeach housed three delusional men claiming to be Christ. It explored identity and belief confrontation.
- Good Samaritan Experiment (1973): Darley and Batson had seminary students rush past a “victim”; hurry reduced helping, showing situational over dispositional factors.
- Piano Stairs (2009): Volkswagen made stairs musical to encourage use over escalators. Part of “fun theory,” it showed gamification boosts healthy behaviors.
- Small World Phenomenon (1967): Stanley Milgram’s letters reached targets in ~6 steps. This birthed “six degrees of separation.”
- Pit of Despair (1960s): Harlow isolated monkeys in vertical chambers, causing severe depression. It underscored social isolation’s devastation.
- Rat Park (1970s): Bruce Alexander gave addicted rats enriched environments; they chose water over morphine. It challenged addiction as purely chemical.
- Invisible Gorilla (1999): Daniel Simons and Christopher Chabris had viewers count basketball passes, missing a gorilla. It proved inattentional blindness.
- Split-Brain Studies (1960s): Roger Sperry studied severed corpus callosum patients; hemispheres processed independently. This advanced lateralization understanding.
- Doll Test (1940s): Kenneth and Mamie Clark showed Black children preferring white dolls, evidencing internalized racism. It influenced Brown v. Board of Education.
- Third Wave (1967): Ron Jones simulated fascism in class; students embraced it rapidly. It warned of authoritarianism’s appeal.
- Shock Machine Self-Experiment (Recent replications): People prefer self-shock over boredom in waiting rooms. It highlighted aversion to inactivity.
- Voodoo Correlations in Neuroscience (2009): Ed Vul critiqued inflated brain-behavior links. It spurred methodological reforms.
- BBC Prison Study (2002): Reicher and Haslam replicated Zimbardo but emphasized group identity over roles.
- Emophilia Studies (2020s): Links love-proneness to more partners and infidelity, a newer trait.
- Project Pigeon (1940s): Skinner trained pigeons to guide missiles. It showed operant conditioning’s applications (canceled).
- Valins Effect (1966): False heart rate feedback altered attractiveness ratings. It demonstrated misattribution.
- Rotter’s Locus of Control (1966): Internal vs. external control predicts behavior; internals are more proactive.
- Weber’s Law (1834): Just noticeable difference is proportional to stimulus intensity. Foundational in psychophysics.
- Stroop Effect (1935): Naming ink color slower when word mismatches (e.g., “red” in blue). It reveals automaticity vs. control.
- Piaget’s Conservation Tasks (1930s): Children fail to see quantity unchanged by appearance until ~7. Key to cognitive development stages.
- Vygotsky’s Zone of Proximal Development (1930s): Learning with guidance exceeds solo ability. Influenced education.
- Seligman’s Positive Psychology Experiments (1990s): Gratitude visits boosted happiness. Shifted focus to well-being.
- Bargh’s Priming (1996): Elderly words slowed walking. Showed unconscious influences (controversial replications).
- Loftus’ Car Crash (1974): “Smashed” vs. “hit” altered speed estimates and false memories. Impacted forensics.
- Zajonc’s Mere Exposure (1968): Familiarity breeds liking. Explains advertising and preferences.
- Festinger’s Doomsday Cult Infiltration (1954): Believers rationalized failed prophecy. Classic dissonance.
- Bem’s Precognition (2011): Claimed future events influence present (e.g., better recall of studied words). Highly controversial, failed replications highlighted psi research issues.
- Prisoner’s Dilemma (1950) – Pairs choose to cooperate or betray; showed self-interest often beats mutual benefit.
- Schachter & Singer Two-Factor Emotion (1962) – Same adrenaline rush became euphoria or anger depending on social cues.
- Festinger’s Doomsday Cult (1954) – When prophecy failed, believers proselytized harder to reduce dissonance.
- Latane & Darley Smoke-Filled Room (1968) – Alone, people reported smoke fast; with passive others, most stayed.
- Zimbardo’s Time Perspective Prison (1970s) – Past-negative prisoners had worse mental health than future-oriented ones.
- Loftus Misinformation Effect (1974) – Post-event suggestions made eyewitnesses remember things that never happened.
- Kahneman & Tversky Prospect Theory (1979) – People fear losses more than they value equal gains.
- Libet’s Free Will Experiment (1983) – Brain readiness potential appeared 300–500 ms before conscious decision to move.
- Dunning-Kruger Effect (1999) – Incompetent people overestimate their ability; experts underestimate.
- Dunbar’s Number (1992) – Humans can maintain ~150 stable social relationships.
- Bem’s Feeling the Future (2011) – Claimed psi evidence (later failed replication, sparked reproducibility crisis).
- Trolley Problem (1967) – Most pull a lever to kill one and save five, but refuse to push a person directly.
- Batson’s Empathy-Altruism (1981) – High empathy leads to genuine altruism, not just egoistic motives.
- Aronson’s Jigsaw Classroom (1971) – Interdependent group work reduced racial prejudice in schools.
- Piliavin Subway Experiment (1969) – People helped a “drunk” victim less than an “ill” one; race also mattered.
- Cialdini’s Low-Ball Procedure (1978) – Agree to a deal, then change terms; people still comply.
- Ringelmann Effect (1913) – Individuals pull less hard on a rope when part of a larger group.
- Godden & Baddeley Diving Recall (1975) – Divers remembered word lists better underwater if learned underwater.
- Capgras Delusion Studies (1923–) – Patients believe loved ones are impostors; linked to face-emotion disconnect.
- Seligman’s Positive Psychology Interventions (2005) – “Three good things” exercise reliably boosts happiness.
- Ekman’s Universal Facial Expressions (1970s) – Six basic emotions recognized across cultures, even isolated tribes.
- Old’s Deoxyglucose Brain Mapping (1970s) – First way to see which brain areas light up during tasks.
- Baumeister’s Ego Depletion (1998) – Self-control acts like a muscle (later heavily questioned).
- Harlow’s Pit of Despair (1960s) – Total isolation of infant monkeys produced irreversible psychosis.
- Vygotsky’s Zone of Proximal Development (1930s) – What a child can do with help today, they do alone tomorrow.
- Rosenthal & Jacobson Pygmalion Effect (1968) – Teachers’ high expectations boosted students’ IQ scores.
- Cialdini’s List Technique (1980s) – “Even a penny helps” increased donations dramatically.
- Nisbett & Wilson Halo Interview (1977) – People invent reasons for choices they don’t understand.
- Bargh’s Professor Priming (1996) – Reading words about professors temporarily raised trivia scores.
- Milgram’s Lost Letter Technique (1960s) – More letters addressed to liked groups were mailed by finders.
- Triplett’s Social Facilitation (1898) – Cyclists went faster when racing others than alone.
- Cacioppo’s Facial EMG Happiness (1980s) – Tiny smile-muscle activation predicts long-term well-being.
- Moscovici Blue-Green Slides (1969) – Consistent minority influence can sway majority perception.
- Tajfel’s Minimal Group Paradigm (1971) – Random group assignment instantly created in-group favoritism.
- Wason Selection Task (1966) – People excel at spotting rule violations but fail abstract logic.
- Festinger & Carlsmith Forced Compliance (1959) – $1 lie made people believe it more than $20 lie.
- Hubel & Wiesel Visual Cortex Columns (1959) – Kittens raised with one eye sewn shut lost cortical function.
- Sherif’s Autokinetic Effect (1935) – Individuals’ estimates converged when in groups.
- Bandura’s Self-Efficacy with Snakes (1977) – Modeling + guided mastery cured phobias faster than exposure alone.
- Aronson & Mills Effort Justification (1959) – Severe initiation made women rate boring group more positively.
- Berkowitz’s Weapons Effect (1967) – Presence of guns increased aggressive responses.
- Latané’s Alone vs Group Shouting (1970) – People shout less for help when they think others can hear.
- Kohlberg’s Heinz Dilemma (1958) – Revealed stages of moral development across cultures.
- Cialdini’s Hotel Towel Reuse (2008) – “Most guests reuse towels” message worked best.
- Penfield’s Brain Stimulation (1950s) – Direct stimulation evoked vivid memories and déjà vu.
- Ramachandran’s Mirror Box for Phantom Pain (1990s) – Mirror reflection tricked brain into “moving” amputated limb, reducing pain.
- Mischel’s Cognitive Reappraisal in Kids (1970s) – Teaching kids to think “marshmallow is a cloud” helped them wait.
- Zajonc’s Cockroach Audience Effect (1969) – Cockroaches solved simple mazes faster, complex ones slower when watched.
- Nuttin’s Name-Letter Effect (1985) – People prefer letters in their own name (implicit egotism).
- Replication Project: Psychology (2015) – Only ~36% of 100 famous studies replicated, launching the credibility revolution.
These experiments span conditioning, social influence, cognition, development, and more. For deeper dives, explore sources like onlinepsychologydegree for the top 25 influential ones or psychology textbooks. Many have modern ethical replications or extensions.
