If you were faced with an armed robber, would your mind be on the gun or the robber’s face? If you saw a shooting in the middle of a street, what would attract your attention more: The flash of the muzzle when the gun goes off or the actions of the shooter?
Eyewitness accounts regarding the circumstances of a violent crime or the identity of the people involved tend to be weak when there’s a weapon involved. The presence of the weapon is often enough to drive other details straight out of the witness’s memory — particularly when the presence of a weapon is unexpected.
The link between the expectations of the witness and the weapons focus effect is fairly clear. For example, a gun being held on a firing range will likely draw less attention from witnesses because they expected the gun to be there all along. On the other hand, a gun in the hand of a woman robbing a bank may occupy a great deal of the witnesses’ attention both because it is unexpected and violates cultural expectations about women and weapons.
This “weapons focus effect” has been acknowledged in forensic circles since at least the late 1970s — yet it is rarely mentioned in court despite the impact it may have on the way that witnesses remember crimes. It can cause them to remember the gun used in a violent crime quite clearly — while only retaining a vague memory of the person holding that gun.
This is just one of the many things that can make eyewitness testimony surrounding violent crimes unreliable. Experienced defense attorneys understand the inherent weaknesses in eyewitness testimony and know how to make those weaknesses visible to the jury to create reasonable doubt.