In many criminal investigations police may, at some stage, ask an eyewitness to view either a live or photo-spread lineup to see if a positive identification of a suspect can be made. The witness's response on such an identification test can have a number of different consequences. If the witness makes a positive identification of the police suspect, the likelihood of the suspect being prosecuted will increase. A positive identification is also likely to make the prosecution case more persuasive to a jury, thereby increasing the chances of a guilty verdict. If, however, the witness rejects the lineup, either because he or she is too unsure to make an identification or believes that the offender is not present, the police may revise their theories about the identity of the offender and search for a new suspect. Or they may simply form the view that the witness is not reliable and seek alternative evidence to support their hypothesis about the offender's identity. Either way the witness's behavior at the identification test is likely to exert an important influence on the direction of the police investigations and, indeed, on the outcome of any trial that results.
There are probably two main reasons why the performance of witnesses at identification tests has attracted the interest of researchers. First, the study of eyewitness identification and other eyewitness memory issues provides a real-world context in which to explore, test, and refine theories of the operation of memory processes. The second and probably much more influential reason is the practical import of the now overwhelming body of evidence that demonstrates the fallibility of eyewitnesses when confronted with an identification test. Numerous laboratory and field experiments attest to the often poor performance of eyewitnesses in identification tests—a matter that becomes of particular concern when it is appreciated that, in many cases, the only evidence against a suspect is an identification made by an eyewitness.