Fingerprints and DNA as Evidence

The Secret History of the Future podcast takes topics that feel like state-of-the-art today, and then shows you the parallels to anything from the distant past. The examples are usually unintuitive: who’d imagine driverless cars of today and the start of the automobile era a century ago bring up similar questions!

 

This blog is about an episode on the similarity between fingerprinting (been there for centuries) and DNA (far more recent) in the use of crime solving. Both are considered unique to an individual, and the judicial system treated both as clinching evidence in their heyday (Ok, DNA is still in its heyday).

 

That’s the problem, says the podcast. TV shows and movies have reinforced the belief that they are clinching evidence. But there were errors in matching fingerprint: 1 in 1,000 positive errors (called a match when it wasn’t really a match), and 1 in 20 negative errors (not called a match when it was really a match). Think about how many wrong convictions (or “not guilt” verdicts) such errors would have resulted in…

 

Errors is the common risk between fingerprints and DNA, says the podcast. Many of us don’t realize that DNA can be transferred quite easily. Therefore, finding DNA on something is not cast iron evidence that the person was even there! Then there’s the point that technology today is so advanced that even trace DNA amounts can be picked out and identified. But if DNA can be transferred easily, then a trace finding shouldn’t carry much weightage whereas a significant quantity couldn’t land up by just happenstance. The judicial system, however, doesn’t factor in for this difference between trace v/s significant amounts of DNA. And that would be leading to errors in verdicts. Worse, factoring in for this difference requires people with maths-y/statistical mindsets, and it’s hard to see how judges, let alone juries, can be trained to think in those terms.

 

Further, in case of both fingerprints and DNA, you don’t know when the sample was left. Before the crime? During the crime? Or after the crime? How many judges/juries really understand that point and calibrate for it?

 

Which is why the podcast says that just as fingerprints stop being considered sufficient evidence and had to be supplemented with other evidence, the judicial system needs to stop treating DNA as cast-iron evidence, as “infallible evidence” of guilt that is sufficient by itself.

 

I guess this is the kind of thing that Mark Twain had in mind when he said history doesn’t repeat itself, but it sure rhymes.

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