Heavy-handed and manipulative interview tactics by detectives were blamed for the failures in cases of rape, arson and armed robbery.
The research was carried out with the backing of Sir Paul Condon, the Commissioner of the Metropolitan Police, after a series of cases were thrown out of court. Information obtained from the study has led to the overhaul of the training of police in interview techniques.
The study, done by a psychologist and a Scotland Yard detective, examined transcripts and audio tapes from police interviews from 1991 to 1996 involving 18 serious crimes, including murder. In all cases,taken from forces around Britain, the officers obtained a confession from a suspect who at first denied the crime.
But the researchers found that detectives frequently used intimidation and psychological manipulation to overcome resistance and secure a confession. In six of the 18 cases this led to the judge disallowing the so-called "confessions". In two of the remaining cases, involving allegations of rape and arson, the jury found the defendants not guilty.
Analysis of the cases identified 33 interviewing tactics used by 37 female and male officers. These broke down into six main categories - intimidation, robust challenge, manipulation, question style, appeal, and soft challenge. Officers who made extreme use of intimidation, robust challenge and manipulation were most likely to have their confessions ruled "inadmissible".
Detective Chief Inspector John Pearse, of the Metropolitan Police's criminal intelligence department, who did the research with Dr Gisli Gudjonsson, of the Institute of Psychiatry in London, said: "In some of the serious crime cases we examined we had people who were breaking down and their confessions were later ruled inadmissible."
The report, "Measuring influential police interviewing tactics: A factor- analytic approach", published in the Legal and Criminological Psychology magazine, added that intimidation included implying that the police had more evidence and suggesting the suspect's girlfriend had provided information.
"It also included maximising the serious nature of the offence and a blatant manipulation of detail." In one case "the suspect was subjected to a continuous onslaught which reduced him to tears".
The study concluded: "The police resorted to manipulative and coercive tactics."
The research findings have helped to reshape the way in which police officers are trained to interview suspects. Officers are now being encouraged to make greater use of ethically acceptable tactics and avoid the more extreme brow-beating measures.Reuse content