DNA AT THE SCENE DOES NOT MEAN DNA FROM THE CRIME: A PRACTICAL REVIEW OF TRANSFER, BACKGROUND DNA, CONTAMINATION, AND FORENSIC INTERPRETATION

Modern forensic DNA profiling can provide very strong statistical support for attributing biological material to an individual, while the questions disputed in investigations and courts often concern how, when, and during which activity that material was deposited. This focused critical narrative review evaluates the inferential gap between contributor attribution and crime-level interpretation by integrating evidence on direct and indirect transfer, subsequent transfer, biological reservoirs, persistence, background and prevalent DNA, glove- and packaging-mediated redistribution, contamination, recovery, and activity-level evaluation. Across experimental, operational, and evaluative studies, materially different histories can produce the same source-level DNA observation. DNA quantity, profile completeness, contributor rank, spatial location, and non-detection can inform a comparative evaluation, but none is a stand-alone marker of direct contact, recency, or crime-related deposition. Likewise, a strong source- or sub-source-level likelihood ratio does not automatically provide strong support for an activity proposition. Interpretation should compare explicit competing histories using sufficiently relevant transfer, persistence, prevalence/background, and recovery data while accounting for handling history, contamination opportunities, unknown contributors, and dependence between samples or sites. The review develops a decision framework that starts with the disputed question and preserves the distinction between analytical observation, source attribution, activity evaluation, and ultimate fact-finding. The principal research priorities are realistic proposition-driven datasets, calibrated recovery information, explicit treatment of uncertainty, and reporting practices that keep conclusions at the inferential level supported by the evidence.