How to expose a liar

November 29, 2005

In interrogating suspected perpetrators it’s important to be able to determine who is telling the truth. A new dissertation from Göteborg University in Sweden shows that strategic questioning can considerably enhance the interrogator’s ability to assess the reliability of information.

In many contexts, such as in the justice system, it is important to be able to judge who is lying and who is telling the truth. Such judgments are not least important when it comes to interrogating suspected perpetrators, where guilty parties have obvious motivation to deny their guilt in order to avoid punishment.

Now Maria Hartwig from Göteborg University in Sweden shows that by questioning in a strategic manner in regard to the evidence, interrogators can improve their ability to determine the reliability of statements.

Several decades of research has shown that people are poor judges of whether other people are lying or telling the truth. Both laymen and police officers succeed in doing so just barely better than chance would predict. In her dissertation, Maria Hartwig examines, by a series of experimental studies, whether it is possible to enhance people’s ability to expose lies in interrogations, by making strategic use of the evidence against the suspects.

In her first study she investigated the skills of police officers in determining whether a person is lying or telling the truth, based on interrogations that they planned and carried out themselves. The results showed that the police officers’ interrogation techniques were widely divergent. However, they did not succeed any better than the level of chance in distinguishing lies from truth.

Her second study tested the effect of presenting evidence against the suspect at different stages of the interrogation. It turned out that if the evidence was presented at the end of the interrogation, the suspects tended to provide information that conflicted with the evidence to a great extent. When the evidence was presented late in the interrogation, the questioners performed much better than when the evidence was presented early.

Her third study involved the teaching of trainee police officers to use evidence strategically, that is, not to divulge evidence during the interrogation but nevertheless to ask questions about the evidence. These trainees, and others who did not receive the specific training, were asked to carry out interrogations and then judge the veracity of the suspects’ statements.

Those who had not received special training were mediocre in judging, just as the experienced police officers were in the first study. Unlike innocent suspects, guilty suspects questioned by specially trained interrogators made statements that were clearly in conflict with the evidence. The specially taught trainees managed to make correct judgments of veracity in more than 85% of cases.

Her fourth study focused on the strategies used by suspects to be believed. The results were that liars made use of many different strategies, whereas truth-tellers primarily figured that they would be believed if they simply related what happened.

Source: The Swedish Research Council