In two studies of 140 people, Dr Danielle Polage from the University of Washington has shown that people who have a good imagination can convince themselves, after they have been told to lie, that they are telling the truth.
The findings, presented at the American Psychological Society annual conference in Denver, Colorado, show that for the majority of people, lying about an event strengthened their memory of the truth. However 10 per cent came to believe that the lied-about event was true and subsequently denied they were lying.
"Lying is a form of imagination in which someone creates an alternative reality," said Dr Polage. "For some people the lie becomes so incorporated into their memory that they believe the lie.
"People who make false confessions because they are tired or think they will get off more lightly, live with the lie for a period of time and have to repeat it. These finding show that for a significant proportion of people the lie becomes reality."
People who repeatedly lie about an event, and are then faced with facts that prove they are lying, cannot admit it because they believe their version of events. These people will not display any of the body or facial movements that can betray a liar.
Participants in the study were asked to fill in lifestyle questionnaires. They were asked whether they had ever been lost in a public place, approached by a scary stranger in a park, or bullied before they were 10 years old.
A week later they were told to make up a story that would allow them to answer 'yes' to an event they said had not happened to them. Two weeks later they told their story to a researcher, who asked them detailed questions. They filled in the questionnaires again one week and five weeks later.
Fifty-five per cent had become more confident in the truth of their initial answers, 28 per cent remained the same and 25 per cent were influenced by their own lies. Ten per cent became completely convinced that their lied-about event was true.
"Time had no impact on their lied-about event. In each study, one in ten people were absolutely convinced that their lies were true," said Dr Polage.