Many forensic disciplines require experts to judge whether two complex patterns are sufficiently similar to conclude that both originate from the same source. Studies in this area have revealed that there are a number of factors that affect perception and judgment and that decisions are subjective and susceptible to extraneous influences (such as emotional context, expectation, and motivation). Some studies have shown that the same expert examiner, examining the same prints but within different contexts, may reach different and contradictory decisions. However, such effects are not always present; some examiners seem more susceptible to such influences than do others-especially when the pattern matching is "hard to call" and when the forensic experts are not aware that they are being observed in an experimental study. Studying forensic examiners can contribute to our understanding of expertise and decision making, as well as have implications for forensic science and other areas of expertise.
Many forensic identifications are based on matching a visual pattern left at a crime scene and one from a suspect. However, a large body of forensic trace evidence (such as shoe prints, firearms, tool marks, bloodstains, hair, finger and palm prints, bite marks, handwriting, ear prints, tire marks, etc.) lacks instrumental analysis. In these types of evidence, the "instruments" are, to a large extent, the human expert examiners themselves, who make judgments on the similarity of visual patterns (Cole, 2001; Haber & Haber, 2008; Mnookin, 2008). Such dependence on specialized human visual perception and judgment in expertise (Busey & Vanderkolk, 2005; Gauthier, Williams, Tarr, & Tanaka, 1998) is common in a wide range of domain experts, from radiologists to military fighter pilots (Berlin & Hendrix, 1998; Dror, Kosslyn, & Waag, 1993).
Fingerprint identification is among the most widely used forensic techniques. It is cognitively challenging because no two fingerprint impressions, even from the same source finger, are ever identical; along with intersource differences, there are also intrasource variations. Due to the elasticity of the skin, the pressure applied, the material on which the prints are left, the method of lifting the prints, and a variety of other factors, visual differences are always introduced, even in the best and most ideal cases. And in the real world of forensics, things are far from ideal; the marks left at a crime scene (the latent prints) usually are partial and distorted and include noise.
Thus, the role of expert fingerprint examiners is complex: They do not simply determine whether two images are "identical" but determine whether different images are sufficiently similar to conclude that they originate from the same source. This can be very challenging, because some intersource differences are extremely small, thus producing look-alike prints that are very similar but, nevertheless, originate from different sources (people). Performance levels are reduced, difficulty is increased, and potential problems of false positives arise as distractors become more similar to the target (Ashworth & Dror, 2000; Vokey, Tangen, & Cole, 2009). With the growing use of searchable databases, the potential for such error drastically increases, because it is more likely that one will find a look-alike nonmatch (Cole, 2005; Dror & Mnookin, 2010; Dror, Péron, Hind, & Charlton, 2005).
This is clearly an interesting area in visual cognition, combining issues in perceptual expertise, judgment, and decision making. It is particularly interesting because fingerprint experts have long been touted as infallible (Federal Bureau of Investigation, 1985). On the rare occasions in which errors are found, individual examiners are blamed, attributing the error to incompetence, negligence, or fraud, insisting that, in the hands of competent experts, errors are "virtually impossible" (Ashbaugh, 1994; Cole, 1998, 2005). …