Larry Laudan. Why Presuming Innocence is Not a Bayesian Prior

epanechnikov:

“…the presumption (of innocence) is not (or at least should not be) an instruction about whether jurors believe defendant did or did not commit the crime. It is, rather, an instruction about their probative attitudes.”

Originally posted on Error Statistics Philosophy:

DSCF3726“Why presuming innocence has nothing to do with assigning low prior probabilities to the proposition that defendant didn’t commit the crime”

by Professor Larry Laudan
Philosopher of Science*

Several of the comments to the July 17 post about the presumption of innocence suppose that jurors are asked to believe, at the outset of a trial, that the defendant did not commit the crime and that they can legitimately convict him if and only if they are eventually persuaded that it is highly likely (pursuant to the prevailing standard of proof) that he did in fact commit it. Failing that, they must find him not guilty. Many contributors here are conjecturing how confident jurors should be at the outset about defendant’s material innocence.

That is a natural enough Bayesian way of formulating the issue but I think it drastically misstates what the presumption of innocence amounts to.  In my view, the…

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7 thoughts on “Larry Laudan. Why Presuming Innocence is Not a Bayesian Prior

  1. Thinks says:

    I was surprised to read that the arguments are based on the proposition that juries are asked to believe that the defendant has not committed the crime –until the prosecution unequivocally proves the contrary.

    In my understanding (through my years in a society where this concept is a central building block of morality and justice), the Presumption of Innocence is a Right afforded to each defendant, by which Right, society, the media, the law and the jury are asked to treat the defendant as innocent until proven guilty.

    I refer to the difference between “believing that…” and “treating in a manner consistent with…”

    I think this may not be just semantics and that it may provide a distinction. Which, itself might call for a rewording of some aspects of the argument.

    (Warm greetings from Italy)

    • epanechnikov says:

      I agree that there definitely exists a clear difference between that…” and “treating in a manner consistent with…”. The belief that the one implies the other (and vice versa) is clearly erroneous. That is because the one can definitely be under our control but the other can’t. However I do not see which aspects of the argument do need some rewording. Could you please let me know to which sentences you refer to?

      Warm Greetings too!

      • Thinks says:

        The entire article up to the last paragraph lays out what others have said or assumed. Therefore, nothing needs changing there. The only paragraph directly referring to what the writer thinks (suggests or proposes), is the last one. The first sentence: “As I see it, the presumption of innocence is nothing more than an instruction to jurors to avoid factoring into their calculations the fact that he is on trial because some people in the legal system believe him to be guilty.”

        Here, the writer misses the opportunity to frame the concept squarely on the burden of the prosecution to prove their case, and focuses on the perceptions of the jurist rather than the instruction from the bench which invariably revolves around whether the prosecution have proved their case.

        I would just change the wording of that first sentence of the last paragraph. Although the jurist, without instruction from the bench, may well behave in the way described by the writer, the concept of Presumption of Innocence exists solely for placing the burden of proof on the prosecution.

      • Thinks says:

        …to clarify my point: “As I see it, the presumption of innocence, a concept the roots of which can be traced to the Magna Carta, in that there it took the first daring steps to protect the people from the arbitrary will of the lord-and-master, is nothing more than an instruction to jurors to favor the evaluation of whether the prosecution has proved their case, over and above their own calculations which may be influenced by the fact that some people in the legal system believe the defendant to be guilty”

      • epanechnikov says:

        The article as well as the discussion have some technical implications that might be difficult to see at first sight. The issue at hand is what epistemology can (or should) be applied to legal decision making. Before elaborating further on this theme it would probably be best two emphasize two things. First, it is the jurors beliefs that might send somebody to jail and not the evidence. Evidence should surely influence those beliefs but we have to uncomfortably accept that evidence can only function as an input (and that is by necessity). Second, beliefs are imperfect and subject to error.

        Legal epistemology recognises the need to “have a self-correction device for identifying and revising our erroneous beliefs” (see Error and the Law Exchanges with Larry Laudan Deborah G. Mayo) and investigates what mechanism can play such role. To give you an example, we are somehow familiar with the BARD (beyond all reasonable doubt) standard of evidence. With BARD we want to control the false positive rates (type I error) “and we are willing to absorb a relatively high rate of false negatives (type II error), if necessary, in order to keep false convictions to an acceptable level”. Here the Presumption of Innocence is translated exactly to that. We consider the defendant’s innocence as our initial hypothesis. This hypothesis will only be rejected if the likelihood of observing evidence at least as extreme as the one that was actually observed for an innocent person is very low (lower than a predetermined level).

        This parallels the Statistical hypothesis testing, a methodology widely used to assess the validity of scientific hypothesis. The question now is – what other normative methodological approaches could play a similar role? Is Bayesian inference relevant and if yes to what prior beliefs would the Presumption of Innocence correspond to?

      • Thinks says:

        Thank you. It took some time to respond as I was learning from your references and links.

        I now recognize the article as part of a discussion within a field of science where the study focuses on attributing specific measures to human actions/reactions as they relate to rules that they, themselves are conceptual rather than mathematical. As such no revision would be necessary, contrary to what I suggested earlier, because in this context the wording deals with the method of examination rather than the concept that is being examined.

        There is a deep river that separates the banks of science and of the non-scientific or academic (street wise) approach. Ideally there should always be an effort to build bridges, so that the two may not become “strangers”. However, I recognize that this is sometimes extremely hard to achieve without diluting science into conceptual assumptions –as I did above.

        It is interesting that “Chance” (random event) may be but a “human” way to define something the roots and causes of which are far too complicated and long reaching to be understood as cause and effect.

      • epanechnikov says:

        Apologies for my late response. I’ve been quite busy these days. I hope you did enjoy the reading.

        The discussion deals indeed with the method of examination rather than the concept that is being examined. You are also right suggesting that probability does also represent a measure of our ignorance. You did remind me the post here

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