Much of what the expert psychologist can tell a judge or a jury is related to research outcomes averaged over many individual subjects. There is even a tendency to value empirical evidence proportional to the number of subjects involved, and to disregard data based on few subjects, particularly evidence from single-subject designs. The rationale of using many subjects is, of course, that the outcome becomes less sensitive to the idiosyncrasies of one or a few participants in a small sample. Thus, the reliability of the results obtained in large samples is enhanced, which is the likelihood that other investigators repeating the study will obtain the same outcome. But the validity of the average for application to an individual case is less dependent upon the sample size than on the distance of that particular individual from the average.

Lawyers seem to be aware of this problem. It is not unusual for experts to be confronted with the question: ‘This is all very well as far as your average subject is concerned, but how do you know that Mr. or Mrs. X (the witness, the suspect, or any other person under discussion) does not differ significantly from the group mean?’ And what about the following question: ‘Did you test the vision (or memory, or any other relevant aptitude or characteristic) of Mr. or Mrs. X?’ The answer to the latter question is almost always negative because expert psychologists usually do not, and may not even be allowed to run tests on individuals featuring in a trial. A case in point is the identification of a suspect through recognition of their appearance. An identification expert is almost never allowed to test the vision or memory of the witnesses, particularly not if these witnesses are police officers. Another example is the reliability of the accuser in a sexual abuse case. Even though we know that in this type of case false accusations are by no means rare (cf. chapter 11), experts on issues of memory, who are often consulted in such cases, are seldom allowed to test the memory of the individual bringing such a charge.