“Absence of evidence is not evidence of absence.” –commonly quoted as Carl Sagan; attribution uncertain
Forensic DNA testimony often focuses on what was detected: whose DNA was found, whether a profile is suitable for comparison, and the strength of support for an inclusion. Less attention is often given to what was not detected. Yet, at activity level, the absence of DNA can be probative when DNA would reasonably be expected if a proposed activity occurred. This issue can be poorly addressed in live testimony, where a negative DNA finding may be dismissed as merely “no result” rather than evaluated as part of the totality of the findings.
This presentation will examine how forensic DNA experts should consider and communicate the evidential value of no-DNA findings when the alleged contact, handling of items, assault, or other activities would be expected to transfer biological material. Examples include the absence of a person of interest’s (POI’s) DNA when that DNA might be expected based on the alleged activity, the biological source, or the nature and duration of contact. If an individual is alleged to have carried a firearm to a crime scene, used a knife in a fight, deposited a body fluid, or engaged in extensive handling or physical contact, then the failure to detect that individual’s DNA provide information that should not be dismissed as merely “no result.” Depending on the competing propositions and the case circumstances, such a finding may provide support for the defense proposition, which is critical for the jury to understand.
This presentation will place these issues within the hierarchy of propositions and the likelihood ratio (LR) framework. At the activity level, the question is not simply whether a person is included or excluded as a possible contributor, but whether the observed findings are more probable under one alleged activity than another. This requires considering transfer, persistence, prevalence, recovery, possible background DNA, and the probability of non-detection. Bayesian networks provide a transparent way to encode these expectations and to show how “absence” can have different evidential meaning in different contexts.
The presentation will also address an underappreciated converse situation: the absence of DNA from neutral or expected background contributors may strengthen the prosecution proposition. For example, if a complainant and suspect spent time together in a larger social group, and DNA from one is found under the fingernails of the other, the absence of additional DNA from other social contacts may be unexpected. In that situation, “no DNA from anyone else” is not empty information; the absence of additional DNA from other social contacts may be unexpected and result in a more informative LR.
Using published activity-level evaluations and transfer, persistence, prevalence, and recovery literature, this talk will argue that negative DNA findings should be evaluated, reported, and testified to with the same logical discipline applied to positive associations. Jurors are often asked to understand the significance of DNA that is present. They may also need help understanding the significance of DNA that is absent.
Sometimes, the “absence of evidence” may actually support “evidence of absence.”
