The latest in my ‘Learn Something New Every Day’ series – taken from The Great Courses’ “Trails of Evidence: How Forensic Science Works” (Lecture 28: Human Memory and Eye-witness Accounts)
Because of the importance of witness identification in criminal convictions (and the dire, dire straits for the convicted and legal system when someone innocent is unjustly convicted) there are some ‘best practice’ elements to a quality police line-up.
1. ‘Double-blind’ line-up – not only does the witness not know who in a line-up is the suspect but none of the officers conducting the line-up should know either. This helps eliminate ‘experimenter bias’ and accidental influence over the outcome (like gasps, clucks, held breath, eye-lines, etc).
2. Keep communication to a minimum – don’t ask any leading questions, don’t reinforce/praise right answers or double-check/prompt wrong ones.
3. A witness will always try to pick the person who most closely resembles the criminal they think they remember (even if they’re not sure). Therefore, officers tell the witness that the perpetrator may or may not be in the line-up.
4. Show them the people in the line-up individually to minimise comparisons and guesses and limit the effects of relative judgements (where someone starts making some educated guesses to fill in their own blanks).
5. Show a line-up in which the suspected culprit is completely absent to help check the witnesses knowledge.
6. The non-suspect people in the line-up should resemble the suspect reasonably but be known to be completely innocent.