“Absence of evidence is not evidence of absence.” It’s commonly credited to Carl Sagan, though nobody’s entirely sure that’s right, and forensic DNA testimony has mostly treated it as an unstated rule: no DNA means no information, move on to the next finding.
Kalafut’s talk argues the opposite. If someone is alleged to have carried a firearm, used a knife, or handled an item enough that contact should have left DNA behind, and it isn’t there, that absence isn’t nothing.
Tim Kalafut, of Sam Houston State University, will present the framework at ISHI 37 in a talk called “The Power of No DNA: When Testimony Should Communicate the Information Gained from the Absence of DNA.”
Below, Kalafut gets into how this actually plays out in testimony: the hierarchy of propositions, where Bayesian networks help make the reasoning visible, and what a negative DNA finding needs to sound like on the stand instead of just “no result.”
What first drew you to this case, question, or problem?
Questions about how the DNA got there have been a part of DNA testimony since before DNA testing existed. (I say that because when it comes to “how” the DNA may have got there, DNA is just another form of trace evidence like glass or fibers, and those evaluations date back to the 1960s.) However, sometimes you don’t find any matching DNA. The presence of DNA clearly has interest in the case, as we get asked about it all the time. So why not consider the other side of the coin, and think about what the lack of DNA evidence might support?
What's the single most important idea you want people to walk away with?
When there is an expectation to observe POI DNA given an activity proposed by the prosecution, yet no POI DNA was recovered, the findings generally support the alternative (defense) proposition.
What are common misconceptions this talk addresses?
With apologies to Carl Sagan, most times the absence of evidence is indeed absence of evidence.
What's the most interesting or unexpected thing you found doing this work?
There is a corollary in that there are times if only the POI DNA is found, given certain proposed activities and case information, the lack of DNA from some other person can increase the support the evidence gives to the prosecution proposition.
Who would benefit most from hearing this talk?
Anyone who has thought, “Just because DNA wasn’t found doesn’t mean it didn’t happen.” (Please overlook the fallacy in that statement; that is a statement about “what didn’t happen”.)
What's one thing from this talk people can actually use once they're back at work, whether that's a technique, a case example, or a new way of thinking about the evidence?
The awareness that if you don’t find what was expected given the circumstances of the case (generally, we only have the prosecution perspective), you may need to consider the evidence supports the defense.
What are you most looking forward to at ISHI this year, besides your own session?
Catching up with people – I didn’t get to come last year.
What's your best advice for someone attending ISHI for the first time?
Specifically, I have advice for students. Don’t spend all your time with the other students from your school. Sit at a lunch table with people you don’t know. Meet new people at the bar. Find someone from a laboratory you might want to work at and ask questions. Put yourself out there; you have to “work it” to network.
What do you enjoy doing when you're away from the office/lab?
I love a good road trip. I talked my family into driving across Austria on the “B” roads during our summer vacation this year. It was fantastic! (Why doesn’t the USA have sporty little wagons to drive? And “Save the Manuals!”)