Transfer and persistence of DNA on the hands and the influence of activities performed

During the evaluation of forensic DNA evidence in court proceedings, the emphasis previously placed on the source of the DNA is progressively shifting to the consideration of the activities resulting in its deposition. While direct contact and deposition may be a likely explanation, alternative scenarios involving DNA transfer through a secondary person or medium are important to consider. Here we assessed whether non-self DNA, indirectly transferred via a handshake, could be detected on surfaces contacted by the opposing hand-shaker after 15min, and considered the variables affecting its persistence in subsequent contacts. In general, the depositor of the handprint was the major contributor to DNA profiles collected from handprints placed on glass plates. Minor contributions from the opposing hand-shaker (as a known contributor) were detected at a lower rate, decreasing as the number of contacted items increased post-handshake. Delays in deposition also affected the detection of the opposing hand-shaker, with a 15min delay between handshaking and contact resulting in the reduced presence, and corresponding LRs, of the known contributor. The handprint depositor was excluded from their own handprint on several occasions, including instances where the opposing hand-shaker was not excluded from the same profile. Several factors appeared to strongly influence the detection of both the depositor and contributing individual involved in the handshake. The relative shedding ability of the pair had the largest effect, where good shedders (whether depositor or contributor) could swamp poor to moderate shedders, while the pairing of two moderate or two poor shedders could result in the detection of both individuals. When the deposition of a handprint was delayed, the activities performed by the individual had a substantial effect on the resultant detection of the contributing profile - multiple contacts with the same items increased the likelihood that the known contributor's DNA would be retained and subsequently detected, through the parking and re-transfer of DNA on used items.