Survival of the Fittest? The Origins and Evolution of the Substantial-Similarity Doctrine


The substantial-similarity doctrine had its origins in the nineteenth century, arising as a way of aiding courts in determining whether accident re-creation evidence, either through actual re-creation or analysis of other similar accidents, is relevant and admissible. In recent decades, the doctrine has become an enigma for some courts, in part because its foundational principles had become so well-understood that for many years they were never expressly stated. A small but growing number of courts are replacing this useful tool for evaluating evidence with a near-blanket rule of exclusion that rejects relevant and reliable evidence. A review of the doctrine’s birth and evolution, combined with a series of thought-experiments, helps define the appropriate bounds for the doctrine. The substantial-similarity doctrine, in its original form, is defended against the emerging trends towards blanket rules, both in terms of coherency of the legal system and in terms of product safety.