A probabilistic analysis of cross-examination using Bayesian networks

The legal scholar Henry Wigmore asserted that cross-examination is ‘the greatest legal engine ever invented for the discovery of truth.’ Was Wigmore right? Instead of addressing this question upfront, this paper offers a conceptual ground clearing. It is difficult to say whether Wigmore was right or wrong without becoming clear about what we mean by cross-examination; how it operates at trial; what it is intended to accomplish. Despite the growing importance of legal epistemology, there is virtually no philosophical work that discusses cross-examination, its scope and function at trial. This paper makes a first attempt at clearing the ground by articulating an analysis of cross-examination using probability theory and Bayesian networks. This analysis relies on the distinction between undercutting and rebutting evidence. A preliminary assessment of the truth-seeking function of cross-examination is offered at the end of the paper.