Prosecutor's fallacy  

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Kunstformen der Natur (1904) by Ernst Haeckel
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Kunstformen der Natur (1904) by Ernst Haeckel

The prosecutor's fallacy is a fallacy of statistical reasoning typically used by a prosecutor to exaggerate the likelihood of a criminal defendant's guilt. The fallacy can be used to support other claims as well – including the innocence of a defendant.

The following claim demonstrates the fallacy in the context of a prosecutor questioning an expert witness: “the odds of finding this evidence on an innocent man are so small that the jury can safely disregard the possibility that this defendant is innocent”. The claim obscures that the likelihood of the defendant's innocence, given the evidence found on him, in fact depends on the likely quite high prior odds of the defendant being a random innocent person – as well as the stated low odds of finding the evidence on such a random innocent person, not to mention the underlying high odds that the evidence is indeed indicative of guilt.

At its heart, the fallacy involves assuming that the prior likelihood of a random match is equal to the likelihood that the defendant is innocent. For instance, if a perpetrator were known to have the same blood type as a defendant and 10% of the population to share that blood type, then one version of the prosecutor's fallacy would be to claim that on that basis alone the likelihood of the defendant's guilt is 90%. The actual likelihood would depend on the size of the population with a matching blood type and would likely be much lower.

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Unless indicated otherwise, the text in this article is either based on Wikipedia article "Prosecutor's fallacy" or another language Wikipedia page thereof used under the terms of the GNU Free Documentation License; or on original research by Jahsonic and friends. See Art and Popular Culture's copyright notice.

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