Opinion ID: 2389693
Heading Depth: 1
Heading Rank: 5

Heading: Use of Probabilistic Analysis in Criminal Trials

Text: The fundamental objections to the use of Bayes' Theorem to establish probability of paternity are both mathematical and jurisprudential. The mathematical objections are suggested above: they raise doubts about the validity of applying a formula designed for statistical probabilities to an assessment of proofs by the jury that, although it can be expressed as a probability, is in fact simply a statement of the strength, or weakness, of the jury's belief in a fact, forced into the mold of a statement of probability. The prior probability that is the basis for the Bayes' Theorem calculation is truly no prior probability at all so far as the jury is concerned. Jurors simply believe, at whatever stage of the proceedings they have to make the assessment, that defendant is guilty or not, and have varying degrees of confidence in that belief. If forced to  and they will be, if Bayes' Theorem is admitted  they can express that degree of confidence as a prior probability, the defendant's probability of guilt is 80%, 50% or 10%. The question remains whether Bayes' Theorem, when applied to such a non-statistical probability estimate, is likely to yield reliable results. That is one of the issues the trial court will deal with in its Rule 8 hearing. The jurisprudential objection is different. It says that even if reliable, this factfinding method should not be used by juries except in the most unusual situations, or where the law explicitly requires a calculation of probabilities. In criminal cases, those objections go beyond the possibility of confusing or overwhelming the jury with mathematical complexities. They go to the heart of the jury's function  the finding of guilt beyond a reasonable doubt. These jurisprudential (and other) concerns are set forth in Laurence H. Tribe, Trial by Mathematics: Precision and Ritual in the Legal Process, 84 Harv.L.Rev. 1329 (1971). Writing concededly in reaction to a perceived risk at the time that mathematics was about to take over the jury's factfinding role, Professor Tribe persuasively argued that probabilistic analysis should but rarely be allowed to aid factfinding in criminal trials  and Bayes' Theorem was very much in mind. Although expressly rejecting a per se exclusion of such evidence, his position comes very close to that. Id. at 1354-55. Some of his arguments, expressed as well by others, must be dealt with. One argument notes the possibility that the jury will use the associative evidence  the probability of exclusion  twice. First, having heard defendant has the blood-tissue type that the guilty suspect must have and that only one in one hundred have it, the jury will include that fact in its initial assessment of guilt, i.e., in its determination of the prior probability. When Bayes' Theorem is then applied to that prior probability to reach a conclusion of probability of paternity, the calculation will necessarily again factor in the probability of exclusion, because it is part of the Bayes' Theorem probability of paternity calculation, impermissibly using the exclusionary percentage twice. Id. at 1366-68. Second, because Bayes' Theorem will be introduced in the State's case and because its use depends on the jury's prior-probability finding, the jury inevitably will be impelled to focus, during the State's case, before all of the evidence is in, on the probability of defendant's guilt. Professor Tribe notes the inconsistency of that result with the presumption of innocence, the jury, of course, required to regard defendant as innocent until found guilty beyond a reasonable doubt. Id. at 1368-71. Simply put, the argument is that the use of that calculation during the State's case impermissibly violates the jury's obligation to keep an open mind until all of the evidence is in and deliberations start. Third, the jury is implicitly asked to find defendant guilty beyond a reasonable doubt even though the probability of paternity itself has a quantifiable element of uncertainty and doubt. Id. at 1375. Stated otherwise, even if the probability of paternity is 95%, does our system of criminal justice encourage a jury to find guilt beyond a reasonable doubt when there is a 5% chance that defendant is innocent? Finally, the argument notes the counter-intuitive impact of Bayes' Theorem and the probability of paternity that can result. The ability of the calculation to convert a very low jury estimate based on the facts into an extremely high one after the formula is applied is one such counter-intuitive result. The formula's ability to declare both the defendant and a suspect (with the same blood type) as each having a 95% probability of being the father is another. With such counter-intuitive results persuasively supported by expert testimony, the fear is the jurors will lose sense of the need to use their intuition, common sense, and sense of community values. Tribe likens the process to a return to trial by battle. Id. at 1376-77. Although some of these issues, both mathematical and jurisprudential, may ultimately become issues of law for this Court, we prefer to commit their resolution initially to the trial court where they will be subjected to adversarial testing. We are inclined to believe that appropriate jury instructions can cure all of them, or at least diminish their risk to the point that the advantages of the expert's calculation outweigh these risks, assuming the opinion is otherwise admissible. Given the guidance of the trial court and the argument of competent counsel, we think juries will be able to cope with the complexities and pitfalls of this kind of probabilistic evidence. Although the dispute on this subject presumably continues, we are not dealing here with some abstruse application of mathematics: the probability of paternity opinion is regularly and routinely used in civil cases and apparently favored if not mandated by both our Legislature and the federal government. The probability of paternity opinion is also routinely used by laboratories that perform this blood-tissue testing. We recognize, however, that if Bayes' Theorem as applied to blood-tissue testing is admitted in this case, it is presumably admissible in any criminal case involving such blood-tissue testing.