A Presumption of Innocence, Not of Even Odds

Abstract

Now I know how the Munchkins felt. Here I have been, toiling in the fields of Evidenceland for some years, laboring along with others to show how use of Bayesian probability theory can assist in the analysis and understanding of evidentiary problems.' In doing so, we have had to wage continuous battle against the Bayesioskeptics-the wicked witches who deny much value, even heuristic value, for probability theory in evidentiary analysis.2 Occasionally, I have longed for law-and-economics scholars to help work this field, which should be fertile ground for them.3 So imagine my delight when the virtual personification of law and economics himself, Judge Richard Posner, came down from a star to help till the evidentiary soil with his powerful economic tools.4 And imagine my further delight when his house landed square on the noggins of the Bayesioskeptics. I do not suppose this will be a fatal blow. Like Napoleon's Old Guard, they refuse to surrender-but neither will they die. Still, I appreciate having such an ally. It may seem that I should be very gracious and roll out the welcome carpet to Evidenceland for Judge Posner. But I confess that my graciousness is tempered by the fact that Judge Posner commits a serious error in the use of Bayesian analysis. He asserts that an unbiased fact-finder should begin consideration of a disputed case with "prior odds of 1 to 1 that the plaintiff or prosecutor has a meritorious case."5 Indeed, Judge Posner considers this starting point the essence of the definition of an unbiased fact-finder. I believe that this view is wrong in principle as a matter of probability theory. It is indeterminate and also fundamentally at odds with the presumption of innocence. Finally, it leads to bizarre results that expose Bayesian analysis to gratuitous ridicule from the wicked Bayesioskeptics.

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