The investigation into the murder of Meredith Kercher was contaminated by many common psychological biases. First and foremost among these was a phenomenon that psychologists call "confirmation bias."
This is the tendency to jump to conclusions and then look for evidence that supports those conclusions, while ignoring evidence that contradicts them. This is not a deliberate policy; it is something we all do without realising it, and it takes a lot of effort and training to avoid it. Unfortunately, that training is rarely provided to detectives, even though hindsight bias can lead to tragic miscarriages of justice when it infects their judgments. Italian police seem to have made their minds up that Amanda Knox was guilty very early on in the investigation, and from then on they seem to have interpreted ambiguous information in ways that supported their hunches.
Take Knox's strange behaviour as she waiting to be questioned by police a few days after the murder. Police officers reported that she did cartwheels and the splits in the police station. According to Domenico Giacinto Profazio, the former head of the Flying Squad in Perugia, Miss Knox had a "strange attitude," and sat on her boyfriend's lap. "I told them it was not appropriate," Profazio later told the court. Such behaviour could arise just as well from nervousness and shock as from guilt, yet the police never seem to have considered the more innocent explanation. Like Meursault, the hero of The Outsider by Albert Camus, Knox was condemned for having the wrong emotional reactions, and denounced for being a soulless monster, incapable of remorse.
Then there is the fact that Knox changed her story. But this too can be a sign of anxiety. As Baltimore Sun reporter David Simon noted in his book Homicide: A Year on the Killing Streets: "Nervousness, fear, confusion, hostility, a story that changes or contradicts itself--all are signs that the man in the interrogation room is lying, particularly in the eyes of someone as naturally suspicious as a detective. Unfortunately, these are also signs of a human being in a state of high stress."
When people rely on misleading or irrelevant cues to detect deception, they may become more confident that they have spotted a lie even though they are mistaken. Psychologists have carried out dozens of studies asking people to spot lies and measuring how confident they feel about their judgments. A 1997 review of this research, based on studies with a combined total of almost three thousand participants, found that people's confidence in their judgments bears no significant relationship to their accuracy. The mismatch is one of overconfidence; not one study found any evidence of people having less confidence than was justified.
One of the studies included in this review compared the detection skills of undergraduates with those of new recruits to federal law enforcement jobs and experienced federal law enforcement officers. Both groups of officers were more confident than the students but no better at spotting lies. The new recruits, who had been on the job an average of only five months, were just as overconfident as the older officers, who had more than seven years of experience. This would seem to imply serious consequences for the criminal justice system.
Last but not least, during her interrogation the police asked Knox to imagine what might have happened at her flat had she been there. It was apparently this question that led Knox to think she had seen Patrick Lumumba (Kercher's boss) at the crime scene, and sign a statement in Italian to that effect. Yet psychologists have long known that merely imagining an event can lead to false memories. The phenomenon is known as "imagination inflation," and has been documented in dozens of studies. Unfortunately, law enforcement officials are as ignorant of this phenomenon as they are of confirmation bias. When they ask suspects to imagine their possible role in a murder they do not at first remember, therefore, this very exercise may unknowingly lead the suspect to believe that he or she was really involved. Likewise, when clinical psychologists repeatedly encourage a client to imagine being sexually abused as a child, they may inadvertently foster a memory of something that did not really happen.
Unfortunately, these errors of judgment are all too common in police investigations. One way to reduce their dangerous effects would be to make police officers and detectives more aware of them, perhaps by beefing up the amount of psychology in their training. But until that happens, it would be probably be wise not to talk to the police if they suspect you of committing a crime. If you are innocent, you would be a fool to think that your innocence will be obvious to all, or that the police officers are unbiased and objective. If you are guilty, the police won't necessarily be able to tell, but why risk it?
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