Soon after Adolf Eichmann, so-called "architect of the Holocaust", went on trial for war crimes in 1961, a Yale psychologist did an experiment. Eichmann had claimed in court he was simply following orders in masterminding the mass deportation of Jews to death camps during the second world war. Stanley Milgram wanted to understand how people could be made to do things they knew to be morally wrong.
In Milgram's experiment, there were three roles: experimenter, learner and teacher. Milgram's volunteers believed they had been given their roles randomly, but in fact every volunteer was allotted the role of teacher and was required to give electric shocks to a so-called learner, played by an accountant trained to act for the role. The role of experimenter was performed by a stern man in a lab coat. Each time the learner got a question wrong, the teacher would give them an electric shock, the voltage increasing by 15 volts for each wrong answer.
Some 65% of volunteers gave shocks up to the maximum level, even though they believed such dosages could be fatal and heard screams, seemingly from another room. In reality, none of the learners was electrocuted.
But how could the volunteers behave that way, believing that they had been?
Milgram argued that his findings showed the power of social roles. He set up his experiment so that volunteers didn't feel they were to blame for what they did (in fact, they could have refused). The experimenter said he would take responsibility for what might happen to the learner. He urged the teacher to increase the voltages, saying: "You have no choice, you must continue". Milgram concluded that the fact that the experimenter had an official position (signified by their lab coat) increased the likelihood of compliance.
This was one of the classic experiments of group psychology, though not all have involved duping volunteers into believing they had electrocuted victims. Group psychology has often involved experiments to explain how individuals' behaviours, thoughts and feelings are changed by group pressures.
It is generally thought to have originated in 1898 when Indiana University psychologist Norman Triplett asked children to spin a fishing reel as fast as they could. He found that when the children were doing the task together they did so much faster than when alone. Triplett found a similar result when studying cyclists - they tended to record faster times when riding in groups rather than alone, a fact that he explained because the "bodily presence of another contestant participating simultaneously in the race serves to liberate latent energy not ordinarily available".
More than a century later, social psychology explores how other people make us what we are; how unconscious, sometimes ugly, impulses make us compliant and irrational. Why, for example, do I smoke even though I know it could be fatal? How can there be such a gap between my self-image and my behaviour (this is known as cognitive dissonance)?
Why do high-level committees of supposed experts make disastrous decisions (for example, when a Nasa committee dismissed technical staff warnings that the space shuttle Challenger should not be launched, arguing that technical staff were just the kind of people to make such warnings - this is seen as a classic case of so-called "groupthink")?
Why do we unconsciously obey others even when this undermines our self-images (this is known as social influence)? What makes us into apathetic bystanders when we see someone attacked in the street - and what makes us have-a-go-heroes? What makes peaceful crowds turn into rioting mobs?
Group psychological studies can have disturbing ramifications. Recently, Harvard psychologist Mahzarin Banaji used the so-called implicit association test to demonstrate how unconscious beliefs inform our behaviour. He concluded from his research that the vast majority of white, and many black respondents recognised negative words such as "angry", "criminal" or "poor" more quickly after briefly seeing a black face than a white one. The Wall Street Journal, reporting on Banaji's research under the headline, We're All Racists at Heart, concluded: "The most strenuous measures, whether they be diversity programmes, bureaucratic oversight, accountability or guilt-ridden self-monitoring, cannot guarantee a level playing field." If this is true, how can racism ever be eradicated?
But such snap judgments prompt good as well as bad judgments. As Malcolm Gladwell argued in his 2005 bestseller Blink: The Power of Thinking Without Thinking, experts often make better decisions with snap judgments than they do with volumes of analysis. There were downsides, he conceded: when police shot dead Amadou Diallo in a Brooklyn stairwell they wrongly surmised he had a gun in his hand, and the fact that he was black may well have contributed to their fatally incorrect judgment.
Such heuristics or cognitive shortcuts are hardly new: two decades ago the Guardian ran a TV advertisement that riffed on just such misperceptions. It depicted a skinhead running down the street. He looked as though he was going to attack a businessman, but when the camera pulled back we saw that he was trying to save the man from falling crates. The strapline read: "Because things are not always what they seem."
What the ad suggested was that everybody's ability to what Gladwell calls "thin-slice" (see box, page 34) can be corrupted by their likes, dislikes, prejudices and stereotypes (perhaps even Guardian readers don't really believe in do-gooder skinheads). It is the business of group psychology to understand that corruption.
The nature of conformity
Although social psychology is overwhelmingly a post-war field, its questions are venerable.
In his great book Leviathan, Thomas Hobbes (the 17th-century philosopher who maintained that without a social contract binding humans to laws, the life of man would be "solitary, poor, nasty, brutish, and short"), wondered why it was that "certain living creatures, as bees, and ants, live sociably with another ... and yet have no other direction, than their particular judgments and appetites ... therefore some man may perhaps desire to know, why mankind cannot do the same." Hobbes's answer was that "men are continually in competition for honour and dignity, which these creatures are not". As Philip Ball writes in his book Critical Mass, Hobbes thought humans are different: their thoughts, desires and sensations are complex and provoke them into conflict.
But what Hobbes didn't consider is how the presence and opinions of others make us conform in unexpected and sometimes degrading ways.
The nature of conformism has obsessed social psychologists for decades. In 1951, psychologist Solomon Asch did an experiment in which volunteers were asked to judge the correct length of a line by comparing it with three sample lines. The experiment was set up so that there was an obviously correct answer. But Asch had riddled a group with a majority of stooges who deliberately chose the wrong answer. The pressure of the majority told on Asch's volunteers. He found that 74% conformed with the wrong answer at least once, and 32% did so all the time.
What impulses were behind such conformism? Social psychologists have long considered that we construct our identities on the basis of others' attitudes towards us. Erving Goffman, in The Presentation of Self in Everyday Life (1959), analysed social encounters as if each person was engaged in a dramatic performance, and suggested that each such actor was a creation of its audience.
Through such performances of self we internalise role expectations and gain positive self-esteem. We cast other individuals and groups in certain roles. Such behaviour may make some of us unconscious racists, but it also lubricates the wheels of social life.
French psychologist Serge Moscovici developed what is called social representation theory, arguing that shared beliefs and explanations held by a group of society help people to communicate effectively with one another. He explored the notion of anchoring, whereby new ideas or events in social life are given comforting redescriptions (or social representations). For example, a group of protesters against a motorway might be described demeaningly by the road lobby as a "rent-a-mob", while the protesters themselves might anchor themselves more falteringly as "eco-warriors".
At this point, group psychology overlaps with social anthropology and the study of "tribes".
Following the crowd
Social psychologists have also been long-obsessed by the psychology of crowds. In 1895, French social psychologist Gustave le Bon described crowds as mobs in which individuals lost their personal consciences. His book, The Crowd: A Study of the Popular Mind, influenced Hitler and led many later psychologists to take a dim view of crowds.
After the war, German critical theorist Theodor Adorno wrote of the destructive nature of "group psychology". Even as late as 1969, Stanford psychologist Philip Zimbardo argued that a process of deindividuation makes participants in crowds less rational.
Most recent crowd psychology has not been content to brand crowds necessarily irrational. Instead, it has divided into contagion theory (whereby crowds cause people to act in a certain way), convergence theory (where crowds amount to a convergence of already like-minded individuals) and emergent norm theory (where crowd behaviour reflects the desires of participants, but it is also guided by norms that emerge as the situation unfolds). British psychologists Benewick and Holton in 1984 studied why violent flashpoints happen during mass demonstrations. They argued that when people in a crowd are treated in a way they take to be unfair by, say, the police, this will reinforce the sense of grievance that led them to protest in the first place.
In the age of MySpace, Facebook and online dating, group psychologists are now trying to find out what goes on when we present ourselves to the world online, how we are judged for doing so and how groups are formed online. Other social psychology touches on such voguish areas of research as social physics (which contends that physical laws might explain group behaviour) and neuroeconomics (which looks at the role of the brain when we evaluate decisions and interact with each other), but the age-old concerns remain part of our zeitgeist.
When Rory Kennedy, director of the 2007 documentary Ghosts of Abu Ghraib (which examines the abuse, torture, and murder of detainees at Abu Ghraib prison, Iraq, at the hands of US military police in 2003), was asked why he wanted to make the film, he said he wanted to probe "the psychology of the perpetrators, and why they were motivated to do what they did.
I was expecting that I would hear about their childhoods, that there was an inclination towards violent behaviour, but when I asked each of them why they committed these acts, they all said the exact same thing: 'I did it because I was told to do it.'"
• Obedience to Authority by Stanley Milgram (Pinter & Martin)
•The Presentation of Self in Everyday Life by Erving Goffman (Penguin)
• The Lucifer Effect by Philip Zimbardo (Rider & Co)
• Peoplewatching by Desmond Morris (Vintage)
• The Tipping Point by Malcolm Gladwell (Abacus)
• Blink by Malcolm Gladwell (Penguin)