What does a DNA result actually tell you?

AnalysisNo forensic technique carries as much public authority, and none is as routinely misdescribed. "The DNA matched" is reported as though it answered the case, when it is an answer to a question about two samples. The distance between those is where most of the misunderstanding about forensic evidence lives.

German law lists what may be determined. Section 81e of the Code of Criminal Procedure permits a molecular-genetic examination of material lawfully obtained to establish the DNA identification pattern, parentage and the sex of the person, and to compare those findings with reference material, so far as necessary to investigate the facts.

It then closes the list: Andere Feststellungen dürfen nicht erfolgen; hierauf gerichtete Untersuchungen sind unzulässig — other findings may not be made, and examinations directed at them are inadmissible. The prohibition covers not only reporting other findings but conducting the examination that would produce them.

The exception is narrow and instructive. Section 81e(2) permits examination of found, secured or seized material, and where it is unknown which person trace material came from, findings about eye, hair and skin colour and about age may additionally be made. Where the person is known, the ordinary limits apply. The additional findings are permitted only where there is nobody to compare against, and they stop at externally visible characteristics.

Alongside the legal bound sits the scientific one, and it depends on the sample. The final NIST scientific foundation review of DNA mixture interpretation records that for high-quality single-source samples it has been demonstrated that different laboratories will arrive at the same result regardless of the specific instruments, tests and software used — and that multiple interlaboratory studies over the past two decades have demonstrated that different laboratories can produce a wide range of results when interpreting the same DNA mixtures.

Mixtures are no longer the unusual case. The same review records that in the 1990s a sample needed to contain thousands of cells, such as from a visible blood or semen stain, whereas a profile can now be extracted from the few skin cells someone might leave when handling an object — and that people constantly shed small amounts of DNA and can transfer small amounts, including someone else’s, from one surface to another.

• Common belief: DNA proves guilt.

  In practice: A DNA result addresses whether a sample corresponds to a reference profile. How the material arrived, when, and whether that matters are questions the examination does not address. German law makes the point structurally by listing what the examination may determine and forbidding other findings.

• Common belief: A DNA sample can be analysed for anything investigators want to know.

  In practice: Under section 81e the permitted findings are the DNA identification pattern, parentage and sex, with a narrow additional category for unidentified trace material. Other findings may not be made and examinations directed at them are inadmissible.

• Common belief: DNA evidence is equally reliable whatever the sample.

  In practice: The final NIST review found different laboratories reach the same result on high-quality single-source samples and can produce a wide range of results interpreting the same mixtures. The kind of sample changes the answer.

• Common belief: Finding someone’s DNA on an object means they handled it.

  In practice: The review records that people shed DNA constantly and can transfer small amounts, including someone else’s, from one surface to another. Presence and the account of how it came to be present are different questions.

Two kinds of bound sit on the same technique.

• A legal bound listing permitted findings and forbidding others — Germany, StPO § 81e(1).
• A narrow exception for unidentified trace material, limited to externally visible characteristics and age — Germany, StPO § 81e(2).
• A scientific bound that depends on the sample: reproducible between laboratories on single-source material, a wide range of results on mixtures — NISTIR 8351.

AnalysisA statutory list of permitted findings is a privacy protection as much as an evidential one. It means a sample taken to answer one question cannot lawfully be mined for others, and that the limit does not depend on the restraint of whoever holds the sample.

The scientific limits do parallel work for the defence. Where a published review establishes that laboratories can differ substantially on mixture interpretation, the interpretation itself becomes a proper subject of challenge rather than a technical detail behind a conclusion.

• One statute and one final scientific review are used. DNA databases, retention rules, familial searching and investigative genetic genealogy were not researched and are not described.
• Nothing here describes how any examination is performed, and nothing here supports any claim about a case.

Related: the limits of forensic evidence, evidence integrity and admissibility, and what a criminal investigation is.