DNA After the Gold Standard

DNA evidence still carries the authority of the “gold standard.” But increasingly sensitive testing has changed both the evidence we recover and the questions that evidence can actually answer.

DNA evidence still carries the aura of the “gold standard.” But DNA did not stay the same.

Early forensic DNA testing often required a visible stain of blood or semen. The physical constraints mattered. Samples were more likely to be single-source, or relatively simple mixtures. The core question was often closer to identification: whose biological material is this?

Then the technology became more sensitive. Labs could generate results from smaller and smaller amounts of biological material. That increased sensitivity meant detecting biological material that may be present for reasons much less relevant than the blood or semen stains that defined many early DNA cases. Swabs from guns, clothing, and other handled objects became routine. DNA moved from visible stains toward lower-level, mixed, and more difficult-to-interpret samples.

That changed the scientific problem.

The question was no longer only whether a strong DNA signal supported an association with a person. The question became what to do with partial, low-level, degraded, or mixed DNA results from three, four, five or more people.

At that point, the gold-standard story starts to break down. Interpretation became much more complicated. Analysts had to decide what data was real, what was artifact, what may have dropped out, how many people contributed, and the many possible ways the observed DNA could have arisen from different combinations of contributors. Increasingly, probabilistic genotyping software was developed to model some of that uncertainty rather than requiring analysts to resolve those questions by hand. But a more sophisticated statistical model does not make the underlying sample simple. And when it is "impossible to tell with certainty which alleles are present in the mixture or how many separate individuals contributed to the mixture . . . .," the meaning of the result changes.

A DNA result may sound precise. It may come wrapped in a statistic. It may carry the cultural authority of the “gold standard.”

But forensic DNA typing does not tell us how biological material got there. It does not tell us when it got there. It does not tell us what activity deposited it.

Those are not refinements of the same question. They are different questions.

Forensic scientists describe these questions using a hierarchy of propositions. At the source level, the question is whether a particular person is the source of the body fluid; at the sub-source level, whether the person is the source of the DNA. Neither, by itself, tells us how or when that person's DNA or body fluid came to be on that item. Those are activity-level questions. The DNA profile itself cannot answer them.

The history of forensic DNA is not simply a story of scientific progress. It is also a story of expanding sensitivity, expanding interpretation, and expanding legal consequence.

As the National Institute of Standards and Technology's Scientific Foundation Review for DNA Mixture Interpretation explained, "All scientific methods have limits." More detection does not automatically mean more truth. Sometimes it means more traces, more complexity, and more room to overstate what the science can actually say.