Gene Hunt, the copper from the TV series Life on Mars who batters crooks into submission in the interview room, may not approve. But a cunning new method of dragging the truth from criminals may be on the horizon, thanks to research by university psychologists.

Researchers from the University of Portsmouth claim that the best way to spot a lie is to make the suspect repeat his or her version of events in reverse order.

In a £136,000 project, the researchers worked on the theory that it takes more effort to make up a story than it does to tell the truth. A subject asked to repeat a concocted series of events in reverse order would be under too much of a strain, they claimed, and would make mistakes.

Detectives use many psychological tricks to trip up liars. These betray obvious signals from shifting uncomfortably in a seat, through stumbling over words to failing to make eye contact.

Another interview strategy used, the baseline method, requires investigators to note the way a suspect reacts to small talk before an interview compared with how he reacts to penetrating questions.

Finally there is the behavioural analysis strategy (BAI), in which interviewers compare the body language of liars and those telling the truth to set a list of questions.

Researchers asked 290 police officers to examine the interviews of 255 students who were given true and false details to use in their answers.

Traditional police interview methods were used in the study, and in those that employed the reverse order tactics – described as “cognitive load interviews” – the interviewer asked the suspects to recall a series of events from the most recent backwards.

Officers were less likely to detect the liars when traditional methods were used in the interviews but were more likely to detect lies when the subjects were asked them to recall events in a reverse order.

The researchers, whose study, Interviewing to Detect Deception, was funded by the Economic and Social Research Council, believe that serial criminals are so well versed in police interviews that they know how to dodge the psychological tricks. But the reverse order method imposes an additional mental stress on liars.

Professor Aldert Vrij, one of the researchers, said: “Those [police officers] paying attention to visual cues proved significantly worse at distinguishing liars from those telling the truth than those looking for speech-related cues.

“In another experiment, liars appeared less nervous and more helpful than those telling the truth contrary to the advice of the BAI strategy.

“Certain visual behaviours are associated with lying, but this doesn’t always work. Nor is comparing a suspect’s responses during small talk, and then in a formal interview, likely to be much help.

“Whether lying or telling the truth, people are likely to behave quite differently in these two situations.

“Evidence also suggests that liars are concerned about not being believed, and so are unlikely to come across as less helpful than truthful people during interview. If anything, guilty people are probably even keener to make a positive impression. All of this makes the investigator’s job very difficult.”

Trying the reverse order tactic worked much better. “Unlike truth-tellers, liars tend to tell their stories in a strict chronological time order and diverting from this order may well be too difficult for them to do,” Professor Vrij said.

“Lying takes a lot of mental effort in some situations, and we wanted to test the idea that introducing an extra demand would induce additional cues in liars. Analysis showed significantly more nonverbal cues occurring in the stories told in this way and, tellingly, police officers shown the interviews were better able to discriminate between truthful and false accounts.”

Further research on this method is to be conducted and the full findings will be shared with constabularies, possibly to come up with a new technique for interviewing suspects.

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