for instance, an eyewitness who is presented with a pool of faces in one batch might assume that the suspect is among them, which may not be correct. If the mug shots are presented together at one time and the witness is asked to identify the suspect, the witness may choose the photograph that is most similar to the perpetrator, even if the perpetrator’s picture is not among those presented. Similarly, if the photographs are presented sequentially and the witness knows that only a limited number will be presented, the eyewitness might tend to “identify” one of the last photographs under the assumption that the suspect must be in that batch. (This is also driven by the common bias toward reaching closure.) A series of studies has shown that judges can be subject to errors in judgment resulting from similar cognitive biases.12 Forensic scientists also can be affected by this cognitive bias if, for example, they are asked to compare two particular hairs, shoeprints, fingerprints—one from the crime scene and one from a suspect—rather than comparing the crime scene exemplar with a pool of counterparts.

Another potential bias is illustrated by the erroneous fingerprint identification of Brandon Mayfield as someone involved with the Madrid train bombing in 2004. The FBI investigation determined that once the fingerprint examiner had declared a match, both he and other examiners who were aware of this finding were influenced by the urgency of the investigation to affirm repeatedly this erroneous decision.13

Recent research provided additional evidence of this sort of bias through an experiment in which experienced fingerprint examiners were asked to analyze fingerprints that, unknown to them, they had analyzed previously in their careers. For half the examinations, contextual biasing was introduced. For example, the instructions accompanying the latent prints included information such as the “suspect confessed to the crime” or the “suspect was in police custody at the time of the crime.” In 6 of the 24 examinations that included contextual manipulation, the examiners reached conclusions that were consistent with the biasing information and different from the results they had reached when examining the same prints in their daily work.14

Other cognitive biases may be traced to common imperfections in our reasoning ability. One commonly recognized bias is the tendency to avoid cognitive dissonance, such as persuading oneself through rational argument that a purchase was a good value once the transaction is complete. A scientist encounters this unconscious bias if he/she becomes too wedded to a preliminary conclusion, so that it becomes difficult to accept new infor-




R.B. Stacey. 2004. A report on the erroneous fingerprint individualization in the Madrid train bombing case. Journal of Forensic Identification 54:707.


I.E. Dror and D. Charlton. 2006. Why experts make errors. Journal of Forensic Identification 56(4):600-616.

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