Chapter 3 - A Framework to Interpret “Trace-DNA” Evidence Transfer

Abstract

It is never a good idea to highlight problems without providing the remedy. “Trace-DNA” is valuable evidence provided it is reported by constant reference to the published literature to support inferences. DNA evidence retrieved from underneath fingernails is used as the example. This is the best-researched evidence transfer that is commonly reported in serious violent assaults. Typically, a victim defends him/herself against an assailant and DNA is transferred from the perpetrator. Once a suspect is identified, a common defense will be that the transfer was from innocent transfer via passive social contact several days earlier. It is possible to evaluate the relative likelihoods of the prosecution vs. the defense propositions of transfer and persistence by referring to published experimental evidence. A generalized statement format is described, along with the limitations of evidence that can be adduced. The method explained follows the “scientific method” in order to avoid “speculation” on the part of the scientist. This reduces the chance of cognitive errors, especially confirmation bias.