My research on topics related to psychology and law was inspired in large part by the belief that if judges are going to base their decision on assumptions about human behavior, as they often do, they should be informed by the best existing evidence from psychological science. Thus when I was first asked to be an expert witness, I regarded it as a great opportunity to provide the court with the best available empirical evidence on the question to be decided.

This turned out to be a lot more difficult than I had imagined. When sworn in, a witness is typically asked to swear to tell “the truth, the whole truth, and nothing but the truth.” The whole truth is almost always out of the question. If I could provide a written summary reviewing the research on a fairly narrow question, such as how a prior identification of a person from a series of mug shots influences a person’s later choice when faced with a lineup, I might be able to provide a pretty complete answer, although it would be full of qualifications and admissions that I just don’t know about the effects of numerous third variables that have not yet been studied. Nonetheless, I would be satisfied that I had written a valid scientific summary.