Calculated Risks by Gerd Gigerenzer

Author:Gerd Gigerenzer
Language: eng
Format: epub, pdf
Publisher: Simon & Schuster

The Chain of Inference

One would think that a match between a defendant’s DNA and the DNA found at a crime scene would prove that the defendant is the source of the trace. But a reported match does not guarantee that the defendant is guilty of the crime or even that he or she is the source of the trace. The chain of uncertain inference from a DNA match to the guilt of a specific person is depicted in Figure 10-1. First, a reported match may not be a true match because of laboratory errors, whether human or technical, that produce false positives, just as in the case of HIV testing. Second, a defendant who provides a true match may not be the source of the trace if the match is coincidental; even rare DNA patterns can occur in more than one person, particularly in biological relatives. Third, a defendant who is truly the source of a DNA trace may not have been present at the crime scene if the real perpetrator or someone else deliberately, or unintentionally for that matter, transferred the defendant’s biological material to the scene. In the O. J. Simpson case, for instance, the defense forcefully alleged that some of the blood evidence at the murder scene was introduced by the police. The defense’s success in cutting the inferential chain between “Source” and “Present at crime scene” was critical to Simpson’s acquittal. Finally, a defendant who had been present at the crime scene may not be guilty; he may have left the trace before or after the crime was committed.

Like the physicians, AIDS counselors, and expert witnesses we have met in previous chapters, attorneys and DNA experts tend to talk about the uncertainties involved in DNA fingerprinting in terms of probabilities. As described in the preceding chapter, the random match probability is the probability that a person randomly selected from a population would match the trace evidence as closely as the suspect. The source probability, in contrast, is the probability that the suspect is actually the source of the recovered trace evidence. The guilt probability is yet something else, because, as mentioned above, even if the suspect is the source of the trace, he may not have committed the crime. The confusion of the random match probability with the other two probabilities leads to two errors. The source probability error occurs when someone overlooks the first two steps of the inferential chain and wrongly infers the source probability directly from the random match probability, that is:

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