72% of Individuals Carry Non-Self DNA Under Their Fingernails From Everyday Activity

A new peer-reviewed study finds that foreign DNA under fingernails is a routine feature of everyday life. This may have direct implications for how this evidence is presented and challenged in court.

For years, prosecutors have placed considerable weight on DNA found beneath a complainant’s or defendant’s fingernails as evidence of physical struggle or direct contact. A significant new study shows that assumption is far more fragile than courts have been led to believe.

Published in Forensic Science International: Genetics in late 2025, the study examined DNA collected from underneath the fingernails and hand surfaces of 25 individuals going about their ordinary, unrestricted daily lives. No criminal scenarios were staged. No controlled contact was arranged. Participants simply lived normally and then submitted to sampling. The results are striking, and every criminal practitioner handling cases involving fingernail DNA evidence should take note for when a relevant case crosses their desk.

What the research found

The most concerning finding from this research indicates that only 28% of individuals in the study had exclusively their own DNA detected in their fingernail samples. That means nearly three quarters of ordinary people, after a typical day’s activities, already carry someone else’s DNA under their fingernails, without any struggle or scratching.

• 28% of individuals had only their own DNA under their fingernails
• 72% routinely carried another person’s DNA under their fingernails from everyday activity
• 30% had unknown source DNA specifically under the fingernails — that is DNA that did not originate from partner, parents/children or friends

Four findings that matter in court

1. Fingernails and hands retain DNA differently. The study found meaningful differences in DNA composition between under-nail swabs and surface hand deposits from the same individuals. These sites cannot be treated as interchangeable or assumed to carry equivalent evidential weight.

2. Fingernail DNA frequently forms complex mixtures. Mixed DNA profiles — more than one contributor — are common in subungual (under-nail) samples taken from people in the general population. The presence of a mixture does not, on its own, indicate an activity linked to an offence.

3. Mixture inversions can occur. A mixture inversion is when the major and minor DNA contributors in a mixed profile do not correspond to what the prosecution’s theory predicts. While the study found mixture inversions were relatively rare (with non-self DNA typically presenting as a minor component), they do occur and their existence must be accounted for during interpretation.

4. Everyday activities alone deposits foreign DNA. The participants who had no restrictions on their daily routines accumulated non-self DNA. This was not indirect DNA transfer in a theoretical sense — it was the documented reality for the overwhelming majority of people sampled.

The presence of a person’s DNA under someone else’s fingernails does not, in itself, establish when, how, or in what context that DNA was deposited.

Why the existing assumption has been wrong

The courtroom narrative around fingernail DNA has historically run something like this: if the complainant has the accused’s DNA under their fingernails, they must have scratched or clawed at the accused during a struggle. It is direct, compelling, and intuitive. It is also, this research suggests, frequently unsupported by the science. Three alternative explanations for the presence of non-self DNA under fingernails now have scientific data to support the propositions:

Prior innocent contact. The study found that the majority of non-self DNA detected under fingernails was associated with individuals known to the participants including cohabitants, partners, family members, colleagues. Everyday physical contact is sufficient to transfer DNA that persists under fingernails.

Indirect transfer. DNA does not require direct person-to-person contact to move. It can transfer via surfaces, objects, or intermediate parties. A shared towel, a frequently touched door handle, or grooming of another person can all result in subungual DNA deposits that bear no relationship to any criminal event.

Accumulated background DNA. Perhaps most significantly for defence practice: fingernails accumulate DNA over time from a variety of sources. Without baseline sampling before any alleged incident, it is impossible to determine whether DNA found under a person’s fingernails was present before, during, or after the events in question.

What this means for your practice

Practical implications for defence practitioners:

• Challenge the bare assertion that fingernail DNA equals recent, direct physical contact. The science no longer supports that as a default inference.
• Request disclosure of the full DNA profile analysis — this will allow assessment of the DNA profile and not merely whether a match was found, but the mixture complexity, major/minor contributor ratios, and whether a mixture inversion was observed or excluded.
• Explore the relationship between the complainant and the accused before the alleged incident. Innocent prior contact — including shared spaces, social settings, professional environments — provides a legitimate alternative explanation.
• Consider instructing an independent forensic expert to assist in disclosing the limitations to the evidence.
• In cases where the prosecution relies heavily on fingernail DNA, consider whether the forensic expert’s evidence addresses the alternative of innocent transfer. If it does not, that gap may be exploitable in cross-examination.

Further Reading

• Forensic Science International: Genetics — Source study on subungual DNA
• NIST — DNA Mixtures: A Forensic Science Explainer
• SWGDAM — Interpretation Guidelines (PDF)