Weapon focus: a gun in the room erases the eyewitness's memory of everything else
When a weapon is visible during a crime, eyewitnesses fixate on it and remember far less about everything else, including the perpetrator's face. In a classic 1987 study, Elizabeth Loftus and colleagues showed people a video of a customer handing a cashier either a check or a gun; those who saw the gun stared at it longer and later recognized the person much less accurately. It's a reason courts now treat single-eyewitness identification with caution.