Most of you will have heard of Pavlov and his famous dog, but do you know about some of the other famous psychological tests that have had a lasting impact on the field? 

Below, we look at five of the most well-known 20th century experiments that identified common behavioural traits that are still referred to today. 

1. Pavlov’s dog, 1890s 

Impact: Identified conditioning, one of the most important concepts in psychology, which has informed our understanding of learning and behavioural psychology. 

By: Russian experimental neurologist, psychologist and physiologist Ivan Pavlov. 

Purpose: To see whether dogs, who automatically salivate as an unconditioned response to food, could learn or be conditioned to salivate in response to a different, conditioned stimulus.   

Method: Using food as the unconditioned stimulus to which dogs naturally salivate, Pavlov introduced the sound of a metronome as a neutral stimulus. He then repeatedly exposed the dogs to the sound of the ticking metronome, before immediately presenting them with food.  

Results: The dogs eventually began to salivate after hearing the metronome. The metronome had become a conditioned stimulus, which could provoke salivation as a conditioned response.  

2. Little Albert Experiment, 1920 

Impact: Confirmed that ‘classical conditioning’ (as he termed it) applies to humans and that adult fears may be connected to early childhood experiences. 

By: John’s Hopkins University professor Dr John Watson. 

Purpose: To test whether involuntary or automatic responses to certain stimuli – in this case irrational fear – can be learned by association in humans.   

Method: Hopkins placed a white rat in front of a nine-month-old toddler dubbed ‘Albert B’, who initially showed no reaction. He then made a loud noise every time the rat was shown to Albert.  

Results: After several instances of the noise being paired with the presentation the rat, Albert began to cry and show fear every time the rat appeared in the room. The rat had become a conditioned stimulus, which could provoke fear as a conditioned response.  

3. Asch Group Experiment, 1951 

Impact: Demonstrated the power of social influence on individual behaviour. 

By: Polish-American social psychologist Solomon Asch. 

Purpose: To determine if an individual will conform to a group’s decision, even if they think it is wrong. 

Method: To understand group influence, 50 male students took part in a series of vision tests where they must choose the longest of three lines on a card. In six tests, students acted independently to give their answers. In another 12, however, pre-selected answers were given to all but one student, who was unaware of the situation.  

Results: In the 12 rigged tests, 75 per cent of the ‘naïve’ students conformed to the incorrect answer, whereas less than one per cent of students gave the wrong in the six ‘uninfluenced’ trials. The experiment demonstrated that people change their behaviour in a group setting; some to fit in (normative influence), others because they perceive group members to be more informed (informational influence). 

4. Stanley Milgram Obedience Experiment, 1955 

Impact: Demonstrated humans are conditioned to obey authority even if it goes against their natural morals or common sense. 

By: Yale University psychologist Stanley Milgram. 

Purpose: To assess authority bias – people’s willingness to obey authority figures when ordered to perform acts that conflicted with their morals, something of particular interest following the Second World War. 

Method: Participants playing the role of ‘teacher’ were told to watch another person (in reality an actor) take a memory test in another room. The teacher had to press a button each time the person answered wrong, with the actors pretending to receive an electric shock.  

Results: Even though the actors appeared to be suffering considerably, a high proportion of the teachers continued to press the button, believing they were increasing the voltage, when urged to do so by an authority figure. Of 40 participants, 26 delivered the maximum shocks, while 14 stopped before reaching the highest levels. 

5. Stanford Prison Experiment, 1971 

Impact: Revealed negative impact of power and individuals’ capacity to abuse others in certain conditions. 

By: Stanford professor Philip Zimbardo. 

Purpose: To learn how individuals conform to societal roles. 

Method: A group of 24 male students were randomly assigned as either a prisoner or guard. The prison guards were told to run a simulated inside the basement of Stanford’s psychology department for two weeks. They were given eight-hour shifts and told to treat the prisoners as they would expect to in real life. 

Results: The experiment had to be pulled after six days because it had become too dangerous. However, it proved that both the guards and prisoners quickly adapted to their roles: prisoners became submissive, while the guards harassed the prisoners verbally and physically. The study was highly influential in explaining how good people could do evil things in the right environments.  

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