Misinterpreting eyewitness expressions of confidence: The featural justification effect

How do we know eyewitness statements of confidence are interpreted accurately by others? When eyewitnesses provide a verbal expression of confidence about a lineup identification, such as I'm fairly certain it's him, how well do others understand the intended meaning of this statement of confidence? And, how is this perception of the meaning influenced by justifications of the level of confidence, such as when eyewitnesses say, I remember his chin? The answers to these questions are unknown, as there is no research on how others interpret the intended meaning of eyewitness confidence. Three experiments show that an additional justification of confidence, relative to seeing a confidence statement alone, can increase misunderstanding in others' estimation of the meaning of the expression of confidence. Moreover, this justification-induced increase in misunderstanding only occurs when the justification refers to an observable facial feature and not when it refers to an unobservable quality (e.g., He is very familiar). Even more noteworthy, both Experiments 2 and 3 show that this featural justification effect is strongest when eyewitnesses express absolute certainty in an identification, such as by stating I am positive. When a highly confident assertion is accompanied by a featural justification others will be most likely to misinterpret the intended meaning.