Eyewitness ID testimony differs from other types of forensic testimony in that it is typically elicited from non-experts (i.e., victim, witness, and/or police officer(s) administering a line-up). The testimony is often personal, emotional, and particularly powerful, especially if unchallenged.
Social science research has shown that eyewitness ID is often unreliable. According to the Innocence Project, "eyewitness misidentification is the single greatest cause of wrongful convictions nationwide, playing a role in nearly 75% of convictions overturned through DNA testing." Such findings underscore the point made on page 127 of the National Academy of Sciences Report, Strengthening Forensic Science in the United States: A Path Forward, that "effective use and interpretation relies on scientific knowledge and continuing research."
Variables affecting the reliability and/or validity of an eyewitness ID is often broken up into two parts: 1) General Impairment Factors (Estimator Variables), and 2) Identification Tests (System Variables).
General Impairment Factors (Estimator Variables) are innate and unchangeable factors that affect every person and identification to varying degrees. These include:
Environmental factors (i.e., lighting, distance between witness and assailant, and time).
Stress - negatively affects a witness's observational skills, which is contrary to common notion that stress sharpens a person's focus.
Memory - factors affecting memory include, but are not limited to, the passage of time since the event, confirmation bias, and the stress associated with witnessing or experiencing a traumatic event.
Weapon focus - the presence of a weapon distracts the victim's focus from the assailant.
Identification Tests (System Variables) are changeable and can be controlled by the criminal justice system through the implementation of procedures and following best practices. Important system variables include the type of line-up used and how it was administered.
In April 2010, the Wisconsin Department of Justice (DOJ) created a Model Policy and Procedure for Eyewitness Identification.
Depending on the case, a challenge to an eyewitness identification could take place pretrial, during trial, or both. Regardless, it is important to get into evidence the social science research. This can be done through the learned treatise statute and/or expert testimony.