More than a decade of sustained, vigorous research has resulted in an applied cognitive psychology of eyewitness behaviour that is a rapidly maturing body of knowledge. Several developments now would permit police forces to increase the sensitivity, reliability, and fairness of eyewitness memory retrieval. The juror's task of estimating the effects of certain variables on the accuracy of eyewitness report can now be a much more informed one, the relevant research literature now being relatively clear and consistent. Across studies the effect size of a number of these variables is moderate or greater. Equally important has been the consistent failure to document several results that would be predicted from the common-sense intuition of juror and jurist alike. (1983) published a controversial article in the American Psychologist entitled 'Eyewitness identification: what can a psychologist tell a jury?' Their implicit answer was 'not much'. They did state, a year later, however, that they 'would like nothing better than to be able to rewrite that article in 10 years with the answer being "a lot" ' McCloskey, 1984, p. 1069).
Several years ago, McCloskey and EgethThough 10 years have not passed since the publication of their article, I should like to suggest that apsychologist can now tell police quite a bit, as well. The reliability and consistency of much of this exportable knowledge is respectably high. In a number of instances where the existence of a phenomenon has been demonstrated, the effect size (Cohen, 1977) of a result across studies is moderate (d = S O ) or greater. In several other instances where a meta-analysis has not been performed so as to allow an estimate of the effect size of a variable impacting eyewitness behaviour, the effect has at least been consistently obtained across studies with little in the way of null results having been reported. The consistency of failure to document several phenomena has also been impressive.To facilitate exposition I will use Wells's (1978) system variable/estimator variable dichotomy as an organizational framework. System variables are, of course, those over which the criminal justice systems has control, while estimator variables have to do with parameters connected with the particular criminal incident and individuals involved, and can only be estimated post hoc. I will discuss system variables as they impact on the encoding, storing, and retrieving of eyewitness memories. The emphasis on memory operations seems entirely appropriate in this instance, inasmuch as actions taken or not taken by the criminal justice system have very demonstrable impacts at each of these stages of memory trace processing.Though in some cases estimator variables likewise have been shown to have rather