profiling everyone in the world, but theory and experience suggests that this uniqueness is attainable in forensic typing. Indeed, some scientists would argue that the existing panoply of characteristics is already sufficient to permit unique identification in many cases. For example, it has been suggested that a probability much less than the reciprocal of the world population is a good indication of uniqueness. The committee has not attempted to define a specific probability that corresponds to uniqueness, but the report outlines a framework for considering the issue in terms of probabilities, and it urges that research into new and cumulatively more powerful systems continue until a clear consensus emerges that DNA profiles, like dermal fingerprints, are unique.
Recommendation 6.1. Behavioral research should be carried out to identify any conditions that might cause a trier of fact to misinterpret evidence on DNA profiling and to assess how well various ways of presenting expert testimony on DNA can reduce any such misunderstandings.
Comment. Scientifically valid testimony about matching DNA can take many forms. The conceivable alternatives include statements of the posterior probability that the defendant is the source of the evidence DNA, qualitative characterizations of this probability, computations of the likelihood ratio for the hypothesis that the defendant is the source, qualitative statements of this measure of the strength of the evidence, the currently dominant estimates of profile frequencies or random-match probabilities, and unadorned reports of a match. Courts or legislatures must decide which of these alternatives best meets the needs of the criminal justice system. At present, policymakers must speculate about the ability of jurors to understand the significance of a match as a function of the method of presentation. Solid, empirical research into the extent to which the different methods advance juror understanding is needed.