The Moment of Truth: Probability Theory and Standards of Proof

Specialists in all the fields involved agree that the process of proof and persuasion in judicial proceedings presents problems in the application of probability theory and communication theory. When the broader term "information" is used, the problems coalesce, both in judicial trials and in other human affairs. In jury trials, one focus of this coalescence is the formulation for the jury of issues of fact to be decided by them, and their progress to verdict or disagreement. Although Cicero asserted that probability is the very guide of life,and Thomas Jefferson thought mathematical reasoning and deductions were a fine preparation for investigating the abstruse speculations of the law, it is often asserted that the complexity of judicial fact determinations prevents the application of any probability theory to them, except in the vaguest and most unspecific terms.

The contention that no rigorous assumptions or rules are justified would seem to place on the contender an obligation to make none in his own operations. It is the purpose of this article to show by means of illustration that (1) some highly restrictive axioms have (consciously or unconsciously) been incorporated into the standards of proof for factual issues and the directions for decision; and (2) that these axioms are internally inconsistent, as well as in disagreement with views held in common by leading exponents of probability theory and application.