Recent research in decision-making has demonstrated the "dud-alternative effect"--the tendency to become more confident that a chosen response option is correct if it is surrounded by implausible response options (Windschitl & Chambers, J Exp Psychol 30:198-215, 2004). This finding may be applicable to a lineup task: The presence of duds (i.e., highly dissimilar fillers) may increase a witness's confidence that an identified (non-dud) lineup member is the criminal. Four studies (N = 665) demonstrate that the mere presence of highly dissimilar fillers inflates witnesses' confidence in a mistaken identification (Studies 1-4), provides evidence that this confidence inflation is due to the duds inflating the perceived similarity of the other lineup members to the criminal (Studies 2, 3), and delineates some conditions under which the effect holds (Studies 3, 4). The addition of highly dissimilar lineup members, far from being inert, as is often implicitly assumed, can bias witnesses' confidence reports.