When a witness to a crime points to a photo in a police line-up and cries "That's him!" he, or she, may be wrong, with tragic consequences.

Mistaken identification is the leading cause of all wrongful convictions identified to date. These mistaken identifications have tended to occur when a police suspect is innocent, but is selected from the line-up by the witness.

Dr Rachel Zajac, of the Department of Psychology, has devised an astoundingly simple tool which reduces false identification by child witnesses, who are particularly prone to it. The Wild Card, a photo depicting a silhouetted figure with a question mark superimposed, can be inserted into photographic line-ups to help children to indicate that the perpetrator isn't there.

Children are often reluctant to verbally reject a line-up. "Just presenting them with a group of photos implies that one of those photos is the right one," Zajac says. "They want to choose someone, so we gave them something to choose. Suddenly, the rate of false identifications was dramatically reduced, but, importantly, the Wild Card didn't impair children's ability to identify the target if he was in the line-up."

Zajac has found the police very receptive to ideas that would improve their practice. "This research straddles the boundary between science and real life. The Wild Card has the potential to prevent wrongful convictions and you can't really put a price on that. It's an amazing thing to play a part in."

A Marsden Fund Fast Start grant is currently allowing further investigation, enabling Zajac to examine issues such as how the Wild Card works, whether it is effective with adult witnesses, and whether it still works when there is a long delay between the witnessed event and subsequent identification.