Although much empirical research has been conducted during the last decade evaluating variables that influence the accuracy of eyewitness identification, the majority of studies have focused on the effects of situational variables (see the other chapters in this volume). In contrast, little is understood about the influence of individual differences in personality and their effects on identification.

There are two primary reasons for this dearth of research. First, most researchers in the eyewitness field have earned their degrees in social or cognitive psychology. They continue to do research from these perspectives, neither of which focuses on individual differences, and have historically treated such differences as experimental error (see Cronbach, 1957) to be eliminated with tighter experimental control.

The second reason relates to a belief articulated in a seminal paper by Wells on the difference between system and estimator variables in eyewitness identification (Wells, 1978). System variables are those that can be manipulated in actual criminal cases, such as the structure of lineups, the time lapse between the crime occurrence and identification, or the use of techniques designed to elicit maximal recall of information (compare Geiselman, Fisher, MacKinnon, & Holland, 1985, 1986).

Estimator variables are those that cannot be controlled in criminal cases and the effects of which must therefore be estimated after the fact. Estimator variables include characteristics of defendants such as their race or attractiveness, characteristics of the crime such as its perceived seriousness, and characteristics of eyewitnesses such as individual differences in personality.