The National Academy of Science’s landmark report, Strengthening Forensic Science in the United States, A Path Forward, states on page 7 that (nuclear) DNA is the only forensic method “rigorously shown to have the capacity to consistently, and with a high degree of certainty, demonstrate a connection between evidence and a specific individual or source.”
This is, in fact, a true statement, with some important caveats. Nuclear DNA evidence is unequivocal, provided:
1) There is a single DNA profile present in the sample.
2) A sufficient quantity of genetic material is present in the sample
3) The genetic material in the sample is not too degraded.
4) It’s clear how the evidence arrived at the crime scene.
5) The testing lab makes no errors in its analysis.
6) The sample of genetic material is from a primary transfer, not secondary or tertiary. (Deposited directly by the person indicated by the DNA profile.)
I won’t belabor you with the details of electropherograms, relative fluorescence units, molecular weights, loci, alleles, detection thresholds, or stochastic thresholds. I’m guessing your eyes would just glaze over.
But be aware. For cases in which the biological sample is a mixture of DNA profiles, or if the sample doesn’t contain sufficient genetic material, or if the sample is degraded, you get into the area in which the analyst has to start making judgement calls. And this puts things right back in the same boat with all the other forensic pattern matching evidence – fingerprints, hair and fiber analysis, ballistics & toolmarks, shoe prints, tire tracks, and bite marks – that rely solely upon the individual analyst’s training, knowledge, experience, judgement, … and cognitive biases. A good example of this would be the Amanda Knox case in Italy. You can see our earlier post about this case here, which goes into more of the detail of DNA analysis (including secondary and tertiary transfer).
Please maintain awareness – DNA is trickier than you might think. Just because someone says they have “DNA evidence,” doesn’t mean it’s a ‘slam dunk.’ You really have to dig into the details; and as always, “the devil’s in the details.” The DNA testing lab should provide a “probability of occurrence” statistic which reflects consideration of all of the above provisions. And keep in mind that the lab won’t be able to tell you if they made any errors in their analysis.
In doubtful cases, look at the electropherograms.
Check that peaks are even and not molehills.
The interpretation of mixtures is very complex.