It can be remarkably difficult to determine whether two photographs of unfamiliar faces depict the same person or two different people. This fallibility is well established in the face perception and eyewitness domain, but most of this research has focused on the "average" observer by measuring mean performance across groups of participants. This study deviated from this convention to provide a detailed description of individual differences and observer consistency in unfamiliar face identification by assessing performance repeatedly, across a 3-day (Experiment 1) and a 5-day period (Experiment 2). Both experiments reveal considerable variation between but also within observers. This variation is such that the same observers frequently made different identification decisions to the same faces on different days (Experiment 1). And when new faces were shown on each day, observers that produced perfect accuracy on one day made many misidentifications on another (Experiment 2). However, a few individuals also performed with consistent high accuracy in these tests. These findings suggest that accuracy and consistency are separable indices of face-matching ability, and both measures are necessary to provide a precise index of a person's face processing skill. We discuss whether these measures could provide the basis for a selection tool for occupations that depend on accurate person identification.

Accurate person identification is central to all security, police, and judicial systems. A commonplace method to achieve this is to compare a photo-ID and the face of its purported owner. The critical aspect of this task is to spot cases in which these two instances of a face do not match. Studies of person identification show that these instances often go undetected when mismatches occur regularly in an experiment, but this differs from everyday operations in which identity mismatches are rare. The current study therefore examined whether infrequent identity mismatches are more likely to go undetected by observers. In Experiments 1 and 2, identity mismatches were detected equally under low (2%) and high (50%) mismatch prevalence. This pattern persisted when viewing conditions were optimized for person identification in Experiment 3, by using a card-sorting task in which all face identities could be viewed repeatedly, and also under increased task difficulty, by constraining viewing conditions temporally in Experiment 4. These results imply that the infrequent occurrence of identity mismatches in security settings such as passport control does not impair an observer's ability to detect these important events.

The ability to identify an unfamiliar target face from an identity lineup declines when the target is accompanied by a second face during visual encoding. This two-face disadvantage is still little studied and its basis remains poorly understood. We investigated several possible explanations for this phenomenon. Experiments 1 and 2 varied the number of potential targets (1 or 2) and the number of faces in a lineup (5 or 10) to explore if this effect arises from the number of identity comparisons that need to be made to detect a target in a lineup. We also explored if this effect arises from an uncertainty concerning which is the to-be-identified target in two-face displays, by cueing the relevant face during encoding. In experiment 3 we then examined whether the two-face disadvantage reflects the depth of face encoding or a memory effect. The results show that this effect arises from the additional comparisons that are necessary to compare two potential targets to an identity lineup when memory demands are minimised (experiment 1), but it reflects a difficulty in remembering several faces when targets and lineups cannot be viewed simultaneously (experiments 2 and 3). However, in both cases the two-face disadvantage could not be eliminated fully by cueing the target. This hints at a further possible locus for this effect, which might reflect perceptual interference during the initial encoding of the target. The implications of these findings are discussed.

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