Fingernail samples as DNA reservoirs: Implications for forensic interpretation

DNA recovered from beneath fingernails is frequently presented in violent crime investigations as potentially probative evidence of physical struggle. However, the biological and interpretive complexity of the subungual environment has not been comprehensively synthesized within a structured activity-level framework. This review reconceptualizes fingernails as semi-protected, time-integrated DNA reservoirs characterized by high background prevalence, mixture complexity, and transfer variability. Drawing upon anatomical, experimental, casework, and probabilistic interpretation literature, this review examines the mechanisms by which DNA accumulates beneath fingernails, including primary, secondary, and tertiary transfer, as well as routine social contact and environmental exposure.

Evidence from prevalence and persistence studies demonstrates that foreign DNA beneath fingernails is common in non-criminal contexts and that mixed profiles represent expected rather than exceptional findings. Dominant contributors may be influenced by stochastic amplification effects, inter-individual shedding variability, transfer mechanisms, and persistence dynamics, thereby limiting straightforward inference regarding the timing or mechanism of deposition. The review integrates modern evaluative principles that emphasize the separation of sub-source attribution from activity-level propositions and highlights the interpretive risks associated with transposing conditional reasoning or adopting narrative assumptions in the absence of structured evaluation.

A fingernail-specific hierarchy of propositions and an applied decision matrix for forensic casework are proposed to support proportional interpretation and reporting. Critical research gaps are identified, including the need for controlled prevalence studies across diverse populations, quantitative probabilistic modeling at the activity level, and enhanced practitioner training in structured evaluative reasoning. By shifting the interpretive paradigm from assumption-driven inference toward biologically grounded and probabilistically informed evaluation, this review strengthens the scientific robustness and judicial reliability of fingernail DNA evidence.