Read more: “Judge Mental: Saving justice from the unreliable mind“
Without hard forensic evidence, an investigation can live or die by the information gathered in face-to-face interviews with the chief suspect. So how do you persuade the guilty to confess?
Aggressive interrogation should never be the answer. A host of studies have shown that forceful questioning hugely increases the chances that a suspect will confess to a crime they didn’t commit. “If you pressurise someone, then they can confess simply because they think they can go home,” says Memon. Pretending to have incriminating evidence, in the hope that the culprit will give up the fight, is similarly flawed. Lab experiments involving subjects accused of a minor misdemeanour show that such bluffing is highly likely to lead to a false confession – perhaps because they have a misguided confidence that the truth will emerge over time (). Sometimes, their minds can become so muddled that they believe the accusation ().
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The impact of a false confession can be far reaching. Not only does it weigh heavily in a case, but the expectations it brings can cloud other evidence presented at a trial. Analyses of cases later exonerated by DNA evidence show that the existence of a confession sways everything from the expert forensic analysis to the eyewitnesses’ testimonies ().
Since the early 2000s, police in the UK have been trained to avoid practices that might increase the risk of false confession. Yet they linger elsewhere. In the US, for instance, officers routinely use the controversial Reid method of questioning, which encourages high-pressure interrogation and, in certain situations, bluffing with fake evidence.
The Canadian Mounted Police, meanwhile, is known to orchestrate elaborate covert operations to draw confessions from suspects. The so-called “Mr Big” technique involves setting up a fake criminal gang whose members are all undercover police officers. Someone suspected of a crime is lured into the gang, and encouraged to commit small offences for rewards. After a while, the suspect will be interviewed by “Mr Big” – another undercover officer pretending to be the gang’s leader – for a bigger, better-paid job. Mr Big will encourage the suspect to confess to the crime and describe how it was committed – ostensibly to collect “dirt” as a kind of insurance should things go sour, or with a promise of using his influence to make the suspect’s problems disappear. If the suspect confesses – as they do three-quarters of the time – it is game over, and they are arrested.
“The Canadian Mountain Police orchestrate elaborate covert operations to draw confessions from suspects, including setting up a fake criminal gang”
The subterfuge and promise of a big reward mean that the technique provides fertile ground for false confession, say and colleagues at Saint Mary’s University in Halifax, Nova Scotia, Canada, who outlined their fears in a recent paper (Psychology, Public Policy and Law, ).
It is difficult to quantify the impact of these methods, though the available evidence suggests that eliminating them could significantly reduce the potential for miscarriages of justice. According to the Innocence Project, about a originally hinged around false confessions.
Even gentler lines of questioning pose some problems. Detectives have traditionally relied on behavioural cues, such as fidgeting or avoiding eye contact, to discern who is telling the truth, but these signals can be misleading, since innocent people often act the same way under stress, says , a forensic psychologist at the University of Leicester, UK. He suggests concentrating on a suspect’s words instead. “Liars often contradict themselves,” he says.
In 2004, he joined forces with and at the University of Portsmouth, UK, to investigate this. Using real recordings of suspect interviews, the group asked 99 police officers to decide which suspects were lying and which were telling the truth. “Those who did best were the ones that relied on content cues,” says Bull. Those who relied on signs of shifty behaviour did no better than chance ().