A scientist’s take on scientific evidence in the courtroom

Scientific evidence is frequently offered to answer questions of fact in a court of law. DNA genotyping may link a suspect to a homicide. Receptor binding assays and behavioral toxicology may testify to the teratogenic effects of bug repellant. As for any use of science to inform fateful decisions, the immediate question raised is one of credibility: Is the evidence a product of valid methods? Are results accurate and reproducible? While the rigorous criteria of modern science seem a natural model for this evaluation, there are features unique to the courtroom that make the decision process scarcely recognizable by normal standards of scientific investigation. First, much science lies beyond the ken of those who must decide; outside “experts” must be called upon to advise. Second, questions of fact demand immediate resolution; decisions must be based on the science of the day. Third, in contrast to the generative adversarial process of scientific investigation, which yields successive approximations to the truth, the truth-seeking strategy of American courts is terminally adversarial, which risks fracturing knowledge along lines of discord. Wary of threats to credibility, courts have adopted formal rules for determining whether scientific testimony is trustworthy. Here, I consider the effectiveness of these rules and explore tension between the scientists’ ideal that momentous decisions should be based upon the highest standards of evidence and the practical reality that those standards are difficult to meet. Justice lies in carefully crafted compromise that benefits from robust bonds between science and law.