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What Is the Product Rule Relating to Forensic Science?

The product rule is an oft-debated, deoxyribonucleic acid (DNA) tool in forensics. It uses probabilities to calculate DNA profile frequencies, linking evidence to a suspect. Like all statistical evidence, it's imperfect and often skewed in news reports.

  1. The Calculation

    • Scientists multiply individual allele frequencies and genotype frequencies to reach an overall profile frequency. This incriminates a suspect based on the probability that a person with this genetic makeup left the stain or other evidence in question.

    History

    • The product rule was developed and perfected through the 17th and 18th centuries by mathmeticians Gottfried Leibniz, Isaac Newton, Augustin Louis Cauchy and Maria Agnesi, according to the math department of the United States Military Academy. The original calculation is made up of variables in the form of Latin alphabet letters.

    Misleading Representation

    • Prosecutors or reporters can skew the evidence gleaned from the product rule, says DNA View. They take a ratio (1/7000) and claim there is a 1/7000 chance that anyone besides the suspect left the evidence, when in fact there's much more than DNA evidence involved in a case. These are called the prosecutor's fallacy or the defense attorney's fallacy.

    Court Cases

    • Law professionals use the product rule in court cases that rely on DNA evidence. Murder or rape cases are the most common cases utilizing this method, according to course material from the John Jay College of Criminal Justice.

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