Thread by @ahugheswriter, The second time I went to the Writer's Police Academy, the NC [...]

The second time I went to the Writer's Police Academy, the NC sheriff's department that ran the thing invited a well-known defense expert on DNA to give a talk.

(That year I also got to meet an FBI profiler, which was cool.)

The DNA expert walked in, body language nervous.

I actually do not remember if the expert was a woman or man; I was too rapt on the information being transmitted at that time, between the speech, body language, and the reaction of the crowd. But let's say it was a woman.

Her nerves were notable because they were unexpected.

Pretty much every expert I'd heard from the whole conference--from an arson investigator to EMTs, from the profiler to a human trafficking investigator--had been at ease in the conference. None of them had felt at all unsafe around the writers or the sheriff's dpt folks there.

(The one notable exception being the cop who'd been undercover with the Mafia until a few years ago. He was freaked out by the crowd and by anyone looking at him, as much as he tried to hide it. But it was obvious why this was so.)

Why was the defense DNA expert worried?

As the woman started giving her presentation, she settled down nicely into professionalism and the quiet confidence of someone who knows their shit.

On TV, the expert said, DNA is portrayed like even more accurate fingerprints, but it's not. At all.

Prosecutors want you to believe it's a 10 million to one chance of getting a DNA match BUT this is most often bullshit, and she told us why.

The way that police forensics run DNA is not a full genotype (is this the right word). That would be too time consuming and expensive. Instead, they run a certain number of chromosomes, let's say 5.

(Be nice to me on specific details, this happened eight years ago.)

The machines give you a result which is often a blob showing up on a graph. (Example that looks sorta like what I saw here: http://www.forensicsciencesimplified.org/dna/img/SInglesourcematch.png, though the ones they had in court were fuzzier and less precise.)

Just like in the picture, you get a result as a series of spikes

The result is basically for what kind of gene you have sitting in the spots that the forensics are studying for. You can have several types, but the forensic machines also occasionally pop up a mistake where the type is uncertain or... maybe this type?

So, one, the machines aren't totally 100% accurate, so there's that.

Also, just because you match the gene scan in the five places someone else does isn't enough to say you did it. It's like saying, ha! You have type O blood. Type O blood was found at scene, you're the killer!

Type O blood, for those who are following along, 45 percent of the American population. Even type AB+, which is very uncommon, is a full 3.4 percent of the population, millions of people.

We're all smart enough to know that having same blood type as the killer is not enough.

Now, if Type AB+ blood is found at the scene, the victim is Type O, and the suspect is Type B-, there's quite a row to hoe to say the suspect was there. (Maybe they didn't bleed, for example, but then you've got to explain who did.)

But if everyone is Type O, doesn't matter.

Let's go back to DNA. According to the defense DNA expert, just because you match the, let's say five, spikes on the chart from the DNA profile of the killer doesn't mean that you're the killer. It means you're part of a pool of people who could be the killer.

OR, notably, you're part of the pool of people whose DNA in the sampling areas is similar enough to the killer's that the machine's one mistake happened in a way that pointed to your type of DNA.

See how it gets complicated?

Now, if we're in a locked door mystery and there's ten people and NONE of them but you have DNA even marginally close to the sample, and the sample is definitely the killer, so sorry, game over. (This is pretty much not how real life works though.)

To my knowledge, at least eight to ten years ago, there wasn't good data as to how common different types and subsets of DNA types were. Some could be terrifically common. Some could be very uncommon. We don't (or at least didn't) know.

So treating DNA like a more accurate fingerprint, as a 1,000,000 to one match, rests on the assumption that folks in a small town aren't genetically similar. Which, is that really true? We don't know.

Just because AB+ is uncommon doesn't mean people in the same town don't have.

(And FYI fingerprints are as much of an art as a science. Two experienced techs can read the fingerprint markers differently, which makes matching... interesting.

We also haven't found identical fingerprints yet, but that doesn't mean they aren't out there. Again, don't know!)

The DNA expert gave a great presentation.

BUT I was fortunate or unfortunate enough to be near the edge of seating in the big auditorium, close to two sheriff's police officers. They were putting off a quiet hostility and making quiet "traitor" type comments about the expert.

Halfway through the presentation, when it was clear she knew her stuff and this all meant we couldn't just jail people simply for DNA matches, a curious thing happened.

The sheriff's officers got really, really mad. Not just us-versus-them mad, sports team mad.

But mad like they deserved for anyone they arrested to be convicted, like they were entitled not to be questioned, and like the expert was "destroying all their work" and taking the side of evil people.

Because she had facts they didn't like.

And they weren't just mad, they were disgusted, which is the emotion most correlated with violence, by the way.

My instincts were screaming at me to stay very, very still and quiet.

At the time I didn't understand the context. We were all here to learn about how the system worked, right? Us writers were paying good money, and anyway the sheriff's department had invited this lady. Why get mad because someone pointed out lazy wins? Work harder, right?

But it became super clear at that moment that a significant portion of them weren't here for excellence and doing great work because the community needed it. Those particular ones were angry because their power to throw people in jail had been threatened by someone speaking.

It should be noted that I like people who think, who deescalate, who serve the community. I like people who are clever and who want excellence. And I have met a few cops like that.

But that anger? Yeah. That was scary. It was one of many moments that made me question.

I am very, very, very against bullies. And bullies with power most of all.

NOTE: If anyone reading this has counterevidence for the strength of DNA evidence as actually 1 in 1,000,000, please share! I am very much in favor of learning and updating my knowledge base.

*counterevidence or a counterargument

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