Why eyeswitnesses give false evidence and how we can stop them.

In October 2004, a rumour of a gang fight circulated on an east London estate and a large crowd of around 50 young people gathered to take part. During the melee, one youth received a fatal blow from a sharp object and the attackers fled on their bicycles. This tragedy left one young man dead and another, Sam Hallam, convicted of murder – wrongly, as it later turned out.

Eyewitness evidence lay at the heart of this case. One witness picked Hallam out of a police lineup while another identified him in an interview. Yet both these testimonies were later shown to be false and eventually – after seven years in prison – Sam Hallam had his conviction quashed and he was released. For all the importance of eyewitness testimony to the justice system, witnesses can and do get things wrong. Understanding exactly why could help us reduce the chances of further miscarriages of justice.

One obvious reason why witnesses make mistakes is that the conditions of an event may not produce very strong memories. The Hallam case provides a good example of such poor conditions: the incident unfolded rapidly, a weapon was involved, it was distressing and some of the perpetrators had the hoods of their hoodies pulled up to hide their faces. These so-called estimator variables can affect how reliable a witness’s memory is.